A professional replacement window installation follows a set order: a survey, careful removal of the old frames, fixing and sealing the new ones, then a certificate that proves the work meets Building Regulations. For most homes the fitting takes one to two days. The ordering and manufacturing stages take far longer. If you want the whole job handled, Value Doors supplies uPVC windows and doors either supply-only or fully fitted, with fitted orders delivered in 3-4 weeks. This guide explains what happens at each stage, what you should agree in writing and what moves the price.
TL;DR
- Installation has five stages: quotes and survey, order and manufacture, removal of the old windows, fixing and sealing the new ones, then certification.
- On site, a straightforward window takes around an hour, and a typical three-bedroom house takes one to two days.
- Registered installers can self-certify under Building Regulations, and you should receive a certificate you can show when you sell.
- Installation is commonly around 20% of the window package cost, and access, window style and redecoration can add to it.
- Value Doors offers supply-only uPVC windows (10 working days) and fully fitted ones (3-4 weeks).
Table of Contents
- What a professional installation involves, step by step
- Building Regulations, FENSA and your certificate
- How long does it take?
- What does replacement window installation cost?
- Supply-only or fully fitted: which suits you?
- Perspective
- Ready to order?
- Preparing your home for installation day
- Checking the finished job
- FAQ
- Sources
What a professional installation involves, step by step
The fitting itself is the shortest part of the project. Most of the elapsed time goes on quoting, surveying and manufacturing. Here is the full sequence, with what to look for at each step.
1. Quotes, home visit and survey
The HomeOwners Alliance describes the usual route: get quotes, have a home visit to discuss options, review the formal quote, confirm installation dates, then pay the balance and receive your certificate after completion. Formal quotes typically ask for a 10% or 20% deposit.
The survey is where accuracy is won or lost. Every opening is measured, and the surveyor notes the condition of the cills, the reveals, any lintel or bay supports, and where the windows need to open for escape or ventilation. A measurement error of a few millimetres is cheap to fix on paper and expensive once the frames are made.
Compare quotes like for like. The HomeOwners Alliance advises getting at least three quotes from installers registered with a competent person scheme. Check that each one lists the same glazing specification, the same number of opening sashes, and the same finish.
2. Agree the fitting scope in writing
Before you pay a deposit, the written quote should say:
- how the frame is fixed to the wall;
- how the gap between frame and wall is sealed inside and out;
- whether cills and internal reveals are replaced or left;
- where trickle vents (small ventilation slots in the frame) will sit;
- whether old windows and rubble are taken away;
- what is included for making good and redecorating plaster around the new frames.
This removes most of the disputes that arise after the fitters leave.
3. Order and manufacture
Made-to-measure frames take time to produce. Lead times differ by material and supplier, so ask for a delivery estimate when you order. At Value Doors, supply-only orders arrive in 10 working days and fitted orders in 3-4 weeks. If you are choosing between material types, the supply-only uPVC windows buyer’s guide explains what to measure and specify.
4. Removing the old windows
Old frames are cut free and lifted out with care. The aim is to avoid damaging the wall, lintel and finishes around the opening. The best practice is to fit and seal the new windows the same day the old ones come out, so the house stays secure and weathertight overnight. A good installer plans the day so that no opening is left unprotected when they leave.
One structural point matters here. The FENSA Guide to Compliance warns that most replacement window systems cannot carry load by themselves. Where the old frame was holding something up, such as a bay with poles or a lintel above, the installer must assess it and fit adequate support before the new window goes in.
5. Fixing the new frames
This is the stage where quality is hardest for a homeowner to see afterwards, so it is worth knowing the standard. According to the FENSA guide:
- all four sides should be securely fixed wherever practicable, using through-frame fixings or lug (strap) fixings;
- fixings should go at least 40mm into masonry, or 25mm into timber;
- there should be at least two fixings per jamb (the vertical sides), with intermediate fixings no more than 600mm apart;
- fixings are driven at 90 degrees to the frame;
- the jamb fixings are packed so the frame cannot twist.
Packers are small spacers set behind each fixing. They stop a tightened screw from pulling the frame out of true, which would otherwise show up later as a sash that sticks or will not lock.
6. Sealing and drainage
The same guide asks for a full perimeter seal with appropriate sealant all the way around the frame, and a weather seal, commonly silicone, between frame and sill. A backing material such as closed-cell foam sits in the gap so the external sealant sets to the right depth. When the job is done, the frame’s drainage must be clear and working. Sealant should never block the drainage slots.
External silicone needs time to cure. If you are repainting or rendering around the frames, allow it to set before you start.
7. Certification and handover
Once the windows are in, a registered installer notifies the work and a certificate follows. Keep it with your house papers. The next section explains why.
Building Regulations, FENSA and your certificate
Since 1 April 2002, Building Regulations have applied to all replacement glazing in England and Wales. The practical rules cover energy efficiency, safety glass, escape routes and ventilation.
There are two ways to show compliance. The first is to use an installer registered with a competent person scheme such as FENSA or Certass. The FENSA Guide to Compliance explains that FENSA is a government-authorised scheme that lets registered installers self-certify replacement windows and doors in dwellings under 18 metres. The second is to have the work inspected by building control. The NHIC guide to replacement windows and doors notes that unregistered or DIY projects need Building Regulations approval and inspection, at a charge.
Three points are worth knowing:
- Selling your home. The HomeOwners Alliance describes the FENSA certificate as an essential home-selling document, because buyers’ solicitors ask for evidence of compliance. A replacement copy costs £30 including VAT.
- Guarantees. The same source says a FENSA-approved installer must provide an insurance-backed guarantee, which covers you if the installer stops trading.
- New or wider openings. Creating a new window opening, or enlarging one, always needs Building Regulations approval from a building control body, so that lintels, damp proofing and cold bridging are dealt with properly, as the NHIC explains.
Planning permission is a separate matter. It is generally not needed for like-for-like replacements, but listed buildings and conservation areas have their own rules, so check with your local council first.
The energy performance rules
Energy standards have tightened. Glazing Today’s April 2026 report on the new Approved Document L says replacement windows and fully glazed external pedestrian doors in existing homes must achieve a U-value of 1.4 W/m²K or a minimum Window or Doorset Energy Rating. Protected buildings have a different route, such as single glazing with low-emissivity secondary glazing.
A U-value measures how quickly heat passes through a window or door. The lower the number, the better it insulates. Our guide to what U-value means for doors explains how to read the figures on a product specification, and the same logic applies to windows. When you compare quotes, ask for the whole-window U-value, not just the glass figure.
How long does it take?
There are two clocks running: the time on site and the time from order to finish.
| Stage | Typical timing | Source |
|---|---|---|
| One straightforward window | Around an hour | HomeOwners Alliance |
| Windows replaced per day | At least 3-4 | HomeOwners Alliance |
| Typical three-bedroom home | 1-2 days | HomeOwners Alliance |
| Whole house including doors | 3-4 days | HomeOwners Alliance |
| Value Doors supply-only order | 10 working days | Value Doors |
| Value Doors fitted order | 3-4 weeks | Value Doors |
Wooden windows that need priming and painting take longer than uPVC. Job size, window type and the number of fitters also change the figure. Plan for a day or two of disruption: rooms need clearing near the windows, and furniture and curtains should be moved back from the openings.
What does replacement window installation cost?
Window prices vary widely with size, material, glazing and style, so any single figure for “the cost of new windows” is a rough guide at best. What can be said with confidence is how installation relates to the total.
Self-Build’s July 2026 cost guide reports that about 20% of the price of windows and doors on its own project went on installation. In one example, a supplier quoted around £250 per window to fit four large aluminium windows. The guide adds that it is typically not cost-effective to fit fewer than four windows, because a visit has fixed costs whatever the job size.
The same guide lists what pushes installation up:
- Window style. Standard PVC casements are the most affordable. Flush and bay windows need more detailed installation.
- Access. New or amended scaffolding adds to the total, especially on upper floors.
- Preparation and finishing. Preparing existing openings and redecorating afterwards can add to the bill.
This is why a fixed, itemised quote is useful. If one quote leaves out making good or waste removal, it will look cheaper than it is.
Reading a quote
When you have your quotes in front of you, check these points:
- Is the glazing specification identical across all of them?
- Does each one state the installer’s registration with a competent person scheme?
- Is the deposit stated (10% or 20% is typical), along with when the balance is due?
- Does the quote cover removal of old frames and waste?
- Does it say what happens to cills, reveals and plaster finishes?
- Is the guarantee insurance-backed?
Supply-only or fully fitted: which suits you?
Both routes are legitimate, and the right one depends on who will be doing the fitting.
Fully fitted suits most homeowners and property managers. One order covers the product, the fitting and the sign-off, and you deal with one point of contact. For a landlord or managing agent with several units, a single fitted order reduces the number of trades to coordinate.
Supply-only suits contractors and builders who have their own fitting teams and need frames on a reliable schedule. With a 10 working day delivery on supply-only orders, you can plan a job around a firm date. Interior designers specifying windows for a client project can also use supply-only when the client’s builder is already on site.
If your project also includes doors, the same choice applies. Our composite doors and patio door replacement guide cover the options, and the question of certification for doors is answered in our guide to FENSA certificates for composite doors.
Perspective
Most of what goes wrong with a window project is decided before the fitters arrive. A rushed survey, a vague quote or a mismatch in specification between suppliers will cost more than any difference in the headline price.
That is why we put so much weight on a smooth ordering process and on knowing the product well. Customers should be able to tell what they are buying, how long it will take and what happens next. Our delivery times are 10 working days for supply-only and 3-4 weeks for fitted orders, and we keep them because a firm date is worth more to most customers than a vague promise.
Security matters just as much as appearance. Advanced locking mechanisms are a priority in the products we supply, because a window or door is the part of your home that has to keep working every day for years. Choosing a supplier that offers both supply-only and fully fitted options means your project does not have to change shape if your plans do.
the author
Ready to order?
Value Doors supplies uPVC windows and doors, plus aluminium windows, composite doors, bifold doors and sliding patio doors, either supply-only or fully fitted. Supply-only orders arrive in 10 working days. Fitted orders take 3-4 weeks. Browse the range and place your order at Value Doors.
If your project includes a rear opening, our aluminium bifold doors are listed at £1756-£3072, and our aluminium sliding patio doors at £1077-£2754. For more help with specifying products, see the Value Doors advice section.
Preparing your home for installation day
A little preparation keeps the day short and the finish clean.
- Clear the area. Move furniture, ornaments and curtains away from every window being replaced, and lift rugs near the openings.
- Give access. Fitters need to reach the outside of each window. Move cars, bins and garden furniture that are in the way.
- Protect valuables. Dust and debris are normal during removal, so cover items you cannot move.
- Plan for pets and children. Open frames and tools make the work area unsafe, so keep both clear.
- Check the weather forecast. Windows are fitted and sealed on the same day they are removed, but a dry day makes the exterior sealing easier.
After fitting, expect some plastering and decorating on the inside faces. Whether that is included depends on your quote, which is why the scope should be agreed in writing beforehand.
Checking the finished job
Before you sign off, walk round each window.
- Open and close every sash and operate every handle. Each should move freely and lock firmly.
- Look at the frame edges. The sealant line should be continuous, even and tidy, inside and out.
- Check the frame is straight against the wall, with no visible bowing.
- Look at the sill and the drainage slots. They should be clear of sealant.
- Confirm the trickle vents are where the quote said they would be.
- Ask when to expect your certificate. A registered installer can self-certify the work, and the certificate should state the whole-window U-value, according to one 2026 industry guide.
Raise any problems on the day, while the fitters are still on site.
FAQ
How much does it cost to replace windows in the UK?
Prices depend on the number of windows, their size, the material and the style, so the best approach is to compare itemised quotes. As a guide to the installation share, Self-Build found installation made up about 20% of the window and door package on its project. Bay and flush windows, scaffolding and redecoration all add to the total.
How long does it take to replace a house window?
A straightforward window takes around an hour, and a fitting team can usually replace at least three or four a day. A typical three-bedroom house takes one to two days on site. The wait from order to fitting is much longer because the frames are made to measure.
Do I need a certificate after the windows are fitted?
Yes. Building Regulations apply to replacement glazing, and a registered installer can self-certify the work and issue a certificate. You will need it when you sell your home. A replacement FENSA certificate costs £30 including VAT, according to the HomeOwners Alliance.
How many fixings does a window need?
The FENSA guide asks for at least two fixings per jamb, with intermediate fixings no more than 600mm apart. Each fixing should go at least 40mm into masonry and be driven at 90 degrees to the frame. Packers behind the fixings stop the frame twisting.
Can Value Doors supply my windows without fitting them?
Yes. Value Doors offers both supply-only and fully fitted options. Supply-only orders are delivered in 10 working days, and fitted orders in 3-4 weeks. You can start from the uPVC windows and doors range.
Sources
- FENSA Guide to Compliance
- Replacing Windows: What To Expect, HomeOwners Alliance
- Guide to Replacement Windows and Doors, The NHIC
- Significantly tougher performance standards for windows and doors, Glazing Today
- How Much Do New Windows Cost to Buy and Install, Self-Build
- Whole-window U-value on compliance certificates, Axiom Flux guide
