TL;DR
- A standard UK window project runs six to twelve weeks from order to certified completion, with manufacturing taking up most of that time and fitting concentrated into one to three days.
- The technical survey is where rotten sub-frames, lintel issues and out-of-square openings get caught. A complete quote should explicitly cover scaffolding, disposal, making-good, trickle vents and FENSA or CERTASS certification.
- On the day, expect noise and dust. The installer should lay protective sheeting, fit each frame plumb and square with shims, seal internally and externally, test every operation, and walk you through handover.
- Before paying the final balance, run the paper test on every window, confirm trickle vents are operational, and file the FENSA or CERTASS certificate with your home documentation when it arrives.
Replacing the windows on a UK home is one of the larger projects most homeowners undertake, and one of the few where the work is split across weeks rather than completed in a single day. The journey runs from the first quote visit through the technical survey, manufacturing lead time, installation itself, and the certification that arrives weeks later in the post. Most homeowners only see fragments of this process clearly, and the gaps are where unhelpful surprises tend to live: a quote that did not include scaffolding, a survey that revealed rotten timber sub-frames, an installation day that ran longer than expected because the lintel needed attention.
This guide walks through the entire process honestly, from the first measure to the final certificate, so you know what to expect at each stage and what a competent installer should be doing. By the end, you will know what to look for, what questions to ask, and how to tell the difference between a smooth professional installation and a corner-cut job.
Quick overview: what to expect during window installation
For a typical UK home, the journey from first quote to final certificate runs as follows. The initial quote visit takes around an hour. The technical survey, usually a separate appointment, takes a further hour. Manufacturing lead time is typically four to eight weeks for standard uPVC casements, longer for bespoke aluminium, timber sashes or non-standard sizes. Installation day on most homes is one to three days for a whole-house replacement, depending on the number of windows and any structural work needed. The FENSA or CERTASS certificate arrives by post within four to six weeks of installation completion.
The total project usually takes six to twelve weeks from order to certified completion. The bulk of that time is manufacturing rather than disruption to the home; the actual fitting work is concentrated into one to three days of work for a standard project.
Stage 1: Home survey — what the installer checks
The technical survey is the appointment that converts your sales decision into a manufacturing specification. It is usually carried out by a surveyor or installer rather than a salesperson, and it is the visit that catches the issues that affect both fit and final cost.
The surveyor takes accurate measurements of every opening, recording width, height and depth at multiple points to identify any out-of-square reveals. Diagonal measurements check for squareness, with a difference of more than a few millimetres between the two diagonals indicating an opening that will need careful packing during installation.
Beyond the measurements, the surveyor inspects the existing frames for rot, the brick reveals for spalling or damage, the lintels for any sag, and the sills for water damage. On Victorian, Edwardian and 1930s properties, the original timber sub-frame is often hidden behind decades of paint and filler, and surveyors with experience can usually identify whether replacement is likely to be needed by tapping and probing.
Other practical checks include access for the installation team, parking arrangements for the day, internal access routes (particularly on flats and townhouses with stairs), and any fixtures that will affect the work: alarm contacts, intruder sensors, internal shutters, blinds, curtain poles and radiators below the windows.
The surveyor will photograph the openings and surrounding areas, both for the manufacturing file and as protection against any subsequent dispute about the condition of the property before work began. A good surveyor explains what they have found before they leave, including any issues that may affect the final cost.
Stage 2: Quote and specification — what should be included
The quote that follows the survey is the document that defines what you are paying for. A complete quote should explicitly cover the technical survey before manufacture, window-by-window dimensions and specifications, frame material and profile system, glazing details (including U-values and safety rating), Window Energy Rating, hardware specification (handles, locks, hinges, trickle vents), colour and finish on internal and external faces, removal and disposal of existing windows, sealing internally and externally, making-good of plaster reveals, scaffolding or access platforms where required, FENSA or CERTASS certification, and the manufacturer’s product warranty and installer’s workmanship warranty with their separate durations clearly stated.
If a quote is silent on any of these, ask. A quote that resists detail is telling you something useful about how the installer works. Compare quotes line by line, not headline by headline: a £600 per window quote may genuinely cost less than an £900 per window quote, or it may exclude scaffolding, disposal, making-good and certification that the higher quote includes as standard.
Stage 3: Choosing the right window type and material
Material and style are linked decisions, but they are not the same decision. The right choice flows from the property and how the window will be used.
uPVC casement windows are the mainstream choice for most UK homes built between 1960 and 2000. They deliver strong thermal performance, low maintenance, and a 20 to 25 year service life at typical fitted costs of £600 to £1,800 per window. Standard white is the cheapest option; foiled woodgrain and coloured profiles add 10 to 30 per cent.
Aluminium windows suit contemporary properties and renovations where slim sightlines and bespoke colour matching matter. Modern thermal-break construction means thermal performance is now competitive with uPVC, and aluminium lasts 30 to 40 years. Typical fitted cost is £900 to £2,800 per window. Heritage steel-look aluminium for period-style replacements sits at the upper end.
Engineered timber is the right answer for Victorian, Edwardian, Georgian and listed properties where nothing else looks correct on the elevation. The trade-off is maintenance: timber wants refinishing every five to seven years. Typical fitted cost is £1,200 to £3,500 per window, with sash configurations at the upper end.
For glazing specification, modern double glazing with low-emissivity coatings, argon gas fill and warm-edge spacer bars is the right answer for most UK homes. Triple glazing earns its 25 to 40 per cent premium on passive-house projects, exposed locations, heat pump installations, and north-facing rooms in colder regions. For properties on busy roads or near transport corridors, acoustic laminated double glazing usually outperforms standard triple glazing for noise reduction at lower cost.
If your window replacement is part of a wider home upgrade, it is worth thinking about external doors at the same time. Coordinating both projects can help with matching frame colours, improving insulation, and reducing draughts across the whole property. For homeowners reviewing their options, Brow Windows also offers a range of replacement doors for UK homes that can be considered alongside new windows.
Stage 4: UK regulations and certificates to know
Replacement windows are controlled fittings under the Building Regulations, which means standards apply regardless of who fits them. Understanding the framework protects you from problems when you sell.
Approved Document L sets the minimum thermal performance for replacement windows. The current edition incorporates the 2023 amendments and applies to dwellings in England. The limiting whole-window U-value for replacement windows is 1.4 W/m²K, or a Window Energy Rating of Band B or better. Modern compliant windows comfortably meet both routes.
Approved Document F covers ventilation, and trickle vents (small ventilation openings in the frame) are now required on most replacement windows to maintain background ventilation. They are unobtrusive in practice but worth knowing about, particularly if you preferred the original windows without them.
Safety glazing rules apply to any window glass at low level. Glass within 800mm of the floor, or 300mm beside a door, must be toughened or laminated safety glass. Most reputable manufacturers fit this as standard.
Fire escape requirements apply to certain bedroom windows in homes with sleeping accommodation above ground floor level. The window must provide an unobstructed openable area of at least 0.33 square metres, with a minimum width and height of 450mm. Your installer should confirm which windows are subject to this requirement and specify accordingly.
Certification is handled by FENSA or CERTASS through registered installers under the Competent Person Scheme. A registered installer self-certifies the work and provides a certificate confirming Building Regulations compliance, usually within four to six weeks of installation. The certificate confirms compliance, registers the work with the local council, verifies the guarantee insurance, and is the document conveyancers ask for when you sell. FENSA covers replacement external windows, doors, roof windows and roof lights against the relevant Building Regulations, but does not cover new builds, extensions, conservatories, or repairs that do not include the frame.
Listed buildings and conservation areas have separate considerations. Listed building consent is usually required for window changes on listed properties, and conservation areas with Article 4 Directions may require planning permission for replacement windows. Discuss the project with the local planning authority before ordering if your property falls into either category.
Stage 5: Manufacturing and lead times
Most UK windows are made to measure, which means the time between order and installation is dominated by the manufacturing process rather than installer scheduling.
Standard uPVC casement windows in white or popular foiled finishes typically have a four to six week lead time. Coloured uPVC, aluminium in standard RAL colours, and engineered timber casements typically run six to ten weeks. Bespoke timber sashes, aluminium with custom RAL matching, heritage steel-look windows, and any non-standard sizes can run ten to twelve weeks or longer.
The manufacturing lead time is the part of the project no installer can compress significantly. If you have a wedding, sale completion, or particular weather window in mind, work backwards from the date you need the windows in.
During manufacturing, you should not need to do anything. The installer will contact you to confirm the installation date once the windows are ready, usually with at least a week’s notice and a final confirmation a day or two beforehand.
Stage 6: How to prepare before installation day
The morning of the installation goes more smoothly with a small amount of preparation. None of this is essential, but each step makes the day more efficient and reduces the risk of damage to your home.
Clear access internally. Move furniture, beds and storage away from the windows on the day. Take down curtains, blinds and shutters. The installer will do this if necessary, but having it done in advance saves time and reduces the risk of damage to your window dressings. If your blinds or shutters are designed to be refitted to the new frames, mention this at the survey stage so the installer can size the frames appropriately.
Protect floors and surfaces. The installer will lay protective sheeting through the access route, but lifting rugs, runners and any loose items in the work area provides additional protection. Pictures and mirrors on walls near the windows can be dislodged by the vibration of removing old frames; take them down for the day.
Manage pets and children. Keep dogs, cats and small children in a back room or out of the house entirely on installation day. The work involves open windows, sharp tools, and brief periods where the property is open to the outside. Pets in particular find this combination interesting, and small children find it dangerous.
Plan for ventilation and heating. Each window will be open at some point during the work, which means the room temperature will drop noticeably in winter. Plan for higher heating use that day, or accept that some rooms will feel cold until the work is finished.
Confirm parking and access. The installer’s van needs to be close to the property for unloading windows and removing the old ones. If you live on a street with restricted parking, arrange a temporary suspension or warn the neighbours. On terraces with rear-access only, the access route needs to be clear.
Notify the alarm company. If your existing windows have alarm contacts or shock sensors, contact the alarm company a few days ahead. The contacts will need to be transferred to the new frames, and a competent installer will work with the alarm engineer rather than around them.
Be available on the day. You do not need to follow the installer around, but you should be contactable for decisions if any issues emerge during the work, particularly if the existing reveals or sub-frames reveal unexpected problems.
Stage 7: What happens on installation day
A typical UK whole-house window installation runs across one to three days for a two or three-person team. Here is what the day actually looks like.
Arrival and protection
The team arrives, walks the access route, and lays protective sheeting through hallways and rooms. Skirting boards and adjacent walls are masked where required. Old window sills are protected from impact damage. This takes 30 to 45 minutes and is the first sign of how seriously the installer takes their work.
Removal of old windows
Each old window is taken out in sequence. The sashes come off first, then the frame is cut or eased out, depending on its construction and condition. Older frames are sometimes cut into sections to remove cleanly without damaging the surrounding brickwork or plaster. Each removal takes 15 to 30 minutes per window depending on the configuration and condition.
Inspection of the opening
With the old frame out, the opening is checked for squareness, damp, structural integrity and the condition of the lintel and sill. This is when issues get found, and an honest installer pauses and walks you through anything unexpected before proceeding. Rotten timber sub-frames, spalled brick reveals, or sagging lintels all need addressing before the new frame goes in.
Fitting the new frame
The new frame is offered up to the opening, packed level with shims, and checked for plumb and square in three planes. Once the team is satisfied the frame is true, it is mechanically fixed through the jambs into the surrounding masonry. Fixings are concealed and capped, with the heads sitting flush to the frame. This is the most important single stage of the entire job: a frame that is not square is a window that will never seal properly, no matter how good the glass.
Hanging sashes and fitting hardware
The sashes (the opening parts of the window) are fitted into the new frame, hinges adjusted, and operation tested. A good installer checks the gap around all four sides of each sash and confirms it is even. Handles, locks, multipoint mechanisms, child safety restrictors and trickle vents are fitted and tested. Each piece of hardware is operated and adjusted before the team moves on.
Sealing internally and externally
The frame is sealed externally with a low-modulus silicone in a colour matched to the frame or surrounding masonry. Internally, the gap between frame and plaster is filled with expanding foam, trimmed back, and finished with either trim, plaster bead or architrave. The threshold or sill is sealed against water ingress. Done properly, you should not see daylight or feel any draught around the closed window.
Testing and adjustment
Each window is operated repeatedly to confirm smooth operation, clean latching and full lock engagement. Hinge tension is fine-tuned. Trickle vents are checked. Any minor binding or stiffness is corrected before the team moves on to the next window or packs up at the end of the day.
Clean-up and handover
Protective sheeting is lifted, debris is bagged and taken away with the old windows, and the property is left clean. The team walks you through the operation of the new windows, hands over manuals and warranty paperwork, and confirms when the FENSA or CERTASS certificate will arrive. A reputable installer will offer a free service call in the first year for any minor adjustments needed as the windows bed in.
Stage 8: What can delay or increase the cost of installation
Several issues can extend the work or adjust the final invoice. Knowing about them in advance reduces the chance of surprise.
Rotten timber sub-frames are the most common single cause of delay on older properties. The original timber under the existing frame may need cutting back and replacing before the new frame can sit cleanly. This adds two to four hours per window and may carry an additional charge.
Damaged brick reveals and spalled masonry around the openings need making good before sealing. Minor damage is usually included in standard installations; significant damage may require a builder or additional time.
Lintel issues are uncommon but serious. A sagging or failed lintel above the window opening needs to be addressed before any work continues. This is rare on standard installations but more common on older properties, particularly bay windows where the structural arrangement is more complex.
Damp around existing frames usually indicates a longer-standing issue with the property, such as failed pointing, blocked drainage or rising damp. The new windows will not solve this, and the installer should flag it before fitting so it can be addressed separately.
Bay windows are more involved than single windows because the structural support, weatherproofing and sealing detail are all more complex. Allow extra time and budget for bay configurations.
Access constraints can extend the work. Properties with restricted parking, narrow staircases, or upper-floor windows requiring scaffolding all add cost and time. Scaffolding for an average two-storey house typically adds £400 to £1,500 to a project.
Weather can affect the work. Light rain is usually fine, but heavy rain or strong winds may pause installation while individual windows are out. A conscientious installer will not leave an opening exposed to the elements; they will reschedule if necessary.
Plaster damage is largely unavoidable on older properties where the original frames were heavily foamed in. A good installer minimises the damage and makes good with filler or plaster bead. Significant redecoration of the surrounding walls is usually the homeowner’s responsibility.
Stage 9: Final checks, sign-off, guarantees and certificates
The project is not complete until you have signed off the work and received the paperwork. Before paying the final balance, work through this checklist.
Operate every window. Each sash should open and close smoothly, latch cleanly, and lock decisively. Handles should feel solid; locks should engage with a satisfying click rather than feeling sloppy.
Check the seals visually. The compression seals around each sash should look continuous, intact and slightly compressed when the window is closed. Run a sheet of paper around the perimeter; it should hold firmly when closed in the seal.
Check the glazing. Look for any chips, scratches or contamination on the glass. Confirm the glazing units are correctly oriented, with any low-emissivity coatings facing the correct way (the installer will know).
Inspect the sealing externally and internally. The external silicone should be neat, continuous, and colour-matched. The internal trim or plaster bead should be flush, with no visible gaps around the perimeter.
Confirm the trickle vents are fitted and operational. Each window subject to ventilation requirements should have working trickle vents.
Receive the paperwork. The manufacturer’s product warranty, the installer’s workmanship warranty, the operating instructions and care guide, and confirmation of when the FENSA or CERTASS certificate will arrive should all be handed over at the end of the work. Most reputable manufacturers offer 10-year product warranties on quality windows, and reputable installers offer 10-year workmanship warranties.
The FENSA or CERTASS certificate arrives by post within four to six weeks. Keep this with your home documentation. Conveyancers ask for it when you sell the property, and without it, buyers can demand a retrospective Building Control inspection, which is costly and avoidable.
Stage 10: Aftercare, maintenance and when to report issues
A new window asks very little of its owner, but a few small habits extend its life considerably.
Clean the frames and glass two or three times a year with warm soapy water and a microfibre cloth. Avoid abrasive cleaners, solvents and pressure washers, which can damage finishes and seals.
Lubricate the hinges and lock mechanisms annually with silicone spray rather than oil-based lubricants, which attract dirt over time. The lock cylinder takes graphite powder rather than spray lubricant.
Check the weather seals visually each spring and autumn. Any sections that have torn, hardened or compressed permanently should be replaced before they cause draughts.
Keep the trickle vents clear of dust and debris. Closing them entirely defeats their purpose, but they should not be obstructed.
Report any issues within the warranty period. If a window starts to drop on its hinges, the lock becomes stiff, condensation forms between the panes (indicating a failed glazing seal), or water ingresses anywhere around the frame, contact your installer. Most reputable installers offer a free service call in the first year and respond promptly to warranty issues afterwards.
Common mistakes homeowners make
Five mistakes appear repeatedly in UK window installation projects.
The first is treating installation as just swapping the glass. Window replacement is a structural and weatherproofing job, not just a product purchase. The frame, the seal, the lintel, the reveals and the threshold all matter, and a poor installation can cause problems for years regardless of how good the windows themselves are.
The second is assuming all double glazing is the same. There is a real difference between cheap uPVC with thin profiles and inadequate steel reinforcement and quality uPVC from a reputable manufacturer with multi-chambered profiles, proper reinforcement and quality hardware. The visual difference is small at first; the long-term performance difference is significant.
The third is not keeping the FENSA or CERTASS certificate. Conveyancers ask for it when you sell, and without it, buyers can demand a retrospective Building Control inspection. File it with your home documentation as soon as it arrives.
The fourth is treating trickle vents as optional. They are required by Approved Document F on most replacement windows, and removing or permanently closing them defeats the ventilation strategy that allows the rest of the house to function without condensation and air quality issues.
The fifth is comparing quotes without comparing inclusions. A cheap quote that excludes scaffolding, disposal, certification and making-good ends up costing more than a realistic quote that includes them as standard. Compare line by line.
Frequently asked questions
1. How long does window installation take?
A standard window replacement is typically one to two hours per window for a two-person team, so a whole-house project of eight to twelve windows usually takes one to three days. Properties needing scaffolding, structural work or bespoke installations can extend to four or five days.
2. Do installers measure windows from inside or outside?
Both. The surveyor measures from inside to confirm internal reveal dimensions, and from outside to confirm the external opening and any trim or sill detail. Brick-to-brick measurements are the structural reference for manufacturing, with frame-to-frame on the existing windows providing additional context.
3. Do I need to remove curtains, blinds or shutters?
Yes, ideally before installation day. The installer can do this if necessary, but having it done in advance saves time and reduces the risk of damage. If your blinds or shutters are designed to be refitted to the new frames, mention this at the survey stage so the installer can size the frames appropriately.
4. Will window installation damage plaster, render or tiles?
Some minor disturbance to the plaster around the frame is normal and is made good by the installer with filler or a plaster bead. Significant redecoration of surrounding walls is usually the homeowner’s responsibility. External render and tiles should not be damaged by a careful installer; if they are, the installer is responsible for making good.
5. How much mess and noise should I expect?
Window installation is genuinely noisy and dusty. Expect drilling, sawing and hammering throughout the working day, with brief quieter periods during fitting and sealing. The installer will lay protective sheeting and clean up at the end, but a fine layer of dust through the work area is normal.
6. Can windows be installed in rain?
Light rain is usually fine. Heavy rain or strong winds may pause the work because the openings cannot be left exposed to weather while individual windows are out. A conscientious installer reschedules if necessary rather than rushing the work.
7. What happens if the old frame is rotten?
The rotten timber is cut out and replaced before the new frame goes in. This adds time and may carry an additional charge, but it is essential for a sound installation. A new frame fitted into rotten timber will not seal properly and will fail prematurely.
8. Do I need planning permission?
Usually no, on standard freehold houses outside protected areas. Like-for-like replacement is covered by permitted development. Listed buildings require listed building consent. Conservation areas with Article 4 Directions may require planning permission. Flats and leasehold properties often have additional restrictions in the lease.
9. Do replacement windows need FENSA certification?
Yes. Replacement external windows are controlled fittings under the Building Regulations. A registered FENSA or CERTASS installer self-certifies the work and provides a certificate that conveyancers ask for when you sell. Without it, buyers can demand a retrospective Building Control inspection.
10. What is Part L and why does it matter?
Approved Document L of the Building Regulations sets the minimum thermal performance for replacement windows. The current limit is 1.4 W/m²K whole-window U-value or Window Energy Rating Band B. Compliance is the installer’s responsibility, but it matters to you because the certificate confirms that the work meets the standard.
11. Are trickle vents required?
On most replacement windows, yes, under Approved Document F. Trickle vents are small ventilation openings in the frame that maintain background airflow when the windows are closed. They are unobtrusive in practice and contribute to better indoor air quality and reduced condensation.
12. Should I choose uPVC, aluminium or timber?
uPVC suits 1960s to 2000s homes and budget-conscious projects. Aluminium suits contemporary properties, large glazed openings, and where slim sightlines or bespoke colour matching matter. Engineered timber suits Victorian, Edwardian and Georgian properties where appearance reads correctly and listed building or conservation requirements may apply.
13. Is triple glazing worth it?
For most UK homes, no. Quality double glazing delivers excellent thermal performance for the UK climate. Triple glazing earns its 25 to 40 per cent premium on passive-house projects, exposed locations, heat pump installations, and north-facing rooms in colder regions.
14. Can I keep or refit plantation shutters?
Sometimes, but it depends on the original shutter sizing and the new frame depth. Mention shutters at the survey stage so the installer can size the new frames to accommodate refitting. In many cases, new shutters made to the new frame dimensions are the cleaner solution.
15. What should I check before paying the final balance?
Operate every window, check the seals with the paper test, inspect the sealing internally and externally, confirm trickle vents are fitted and operational, receive the warranty paperwork, and confirm when the FENSA or CERTASS certificate will arrive. Hold final payment until you are satisfied the work is complete and the paperwork is in order.