Fitting a window is not difficult in the way that plastering is difficult. It is difficult in the way that measuring is difficult: every fault that shows up later, a casement that binds, a draught down one side, a frame that bows and will not lock, was put in during the first twenty minutes, when the frame was set in the opening. This guide is the sequence the trade uses, the numbers from the industry good practice guide and FENSA's compliance guide, and the regulations that apply whoever holds the drill. Taking the old window out is covered separately in how to remove a double glazed window. A door and frame goes in differently, with deeper fixings into masonry and a rule on threshold height, covered in how to fit a uPVC door.
Measuring: where most DIY fits fail
Measure the structural opening, brick to brick, never the old frame, because the old frame may have been packed, trimmed or the wrong size from the start. Take the width in three places, top, middle and bottom, and the height in three, left, centre and right, and use the smallest of each; do not average them. Check the opening is square: the industry guide allows 5 mm variation across the widths and heights and 10 mm across the diagonals before the opening itself needs work.
Then deduct for fitting tolerance, and deduct once, not per side. For white uPVC the GGF good practice guide takes 10 mm in total off the width and off the height for frames up to 3 m, 15 mm up to 4.5 m and 20 mm beyond. Coloured and foiled uPVC expands more in sun, so it takes 15, 22 and 28 mm. That leaves roughly 5 mm each side for packers, which is what you want. Two more things to record: whether there is a check reveal, where the frame sits behind the outer brickwork, and how the sill will work, because the sill arrangement changes the manufactured height. A sill should overhang the face of the wall by at least 25 mm. Our guide on how to measure windows for replacement covers check reveals, bays, sashes and cills step by step, and who pays when a supply-only size is wrong; our installation process guide covers the survey a professional does at this stage.
The sequence
1. Sill first
Run a continuous bead of silicone along the masonry where the sill will sit, silicone the ends of the sill and push the end caps on, and fit any expansion gaskets the system uses. Bed the sill without blocking its drainage holes. Level it; everything above is set from this line.
2. Level, plumb, square
Sit the frame on the sill. Level it with plastic packers under the bottom rail, plumb the jambs, then check the head. Measure the diagonals: equal diagonals mean square. Sight across the face for twist. Then place a packer behind every point where a fixing will go, because a screw tightened into a frame with air behind it pulls the frame into the wall and bows it, and a bowed uPVC frame is one whose openers never close cleanly again. Work from the sill up, alternating sides so the frame stays balanced.
3. Fix it
FENSA's compliance guide sets the fixing rules the trade works to: fix all four sides wherever practicable, through the frame into the wall or with lugs and straps screwed to the frame before it goes in; at least two fixings per jamb, intermediate fixings at no more than 600 mm centres, at right angles to the frame, and packed to prevent twist; and fixings that penetrate at least 40 mm into masonry, or 25 mm into a timber-framed wall. The system houses add that corner fixings sit 150 to 200 mm from each corner and no fixing within 150 mm of a mullion or transom centre line. Pre-drill the frame on the bench, put a dab of silicone under each screw head, and do not over-tighten: recheck level and plumb after each fixing. Sound brickwork takes a masonry screw directly; poor brick wants a plug or a lug.
4. Glass back in, toe and heel
Refit the sealed units on their packers, two along the bottom and one or two up each side, set so the weight of the glass squares the sash: on an opener that means diagonally from the bottom hinge corner to the top lock corner, the toe-and-heel packing the system specifies. This is what keeps a casement from dropping and binding in a year's time. Clip the beads back with a rubber mallet, never a hammer; the trade disagrees on whether long or short beads go first, and agrees that the last one is tapped home at an angle.
5. Seal it
Fill the perimeter gap from inside with low-expansion gun-grade foam, and do not over-fill: foam that is packed too tight bows a frame as surely as an unpacked fixing. Foam insulates and seals; it does not hold the window in, and a frame relying on foam alone is one of the named installer failures. Outside, a continuous bead of low-modulus exterior silicone where frame meets masonry, including under the sill, tooled smooth, and the same inside. Leave the drainage slots on the outside bottom of the frame clear; sealing them traps water in the frame and is the commonest cause of an early failed unit. Trims inside, a square bead across the head and quad down the sides. Take the protective tape off the frame the same day; sun bonds it on.
6. Test it
Open and close every sash fully, lock and unlock every handle, and run a hand round the frame on the first windy day. Crooked trims, a sash that scrapes, or a fat bead of silicone doing the job foam should have done behind it are the signs of a poor fit that a surveyor lists when the house is sold, and they are all visible on the day.
The packer behind the fixing is the whole trade in one part. A frame fixed without one looks fine on the day and binds by the first summer. It costs pennies and it is the step DIY fits skip most.
Double Glazing Cost Calculator Team
The regulations you take on
Replacing a whole window is a controlled fitting under the Building Regulations in England and Wales. A FENSA or Certass registered installer self-certifies and issues the certificate; anyone else, including you, must give a building notice to the local authority before starting, be inspected and pay for a completion certificate. Councils set their own fees: for 2026/27 a notice for replacement windows in a house runs about £220 to £370, some charging a base fee plus £20 a window, and regularising work after the event costs more, about £356 to £445. A registered fitter's certificate is about £25 a window by comparison. Widening the opening is outside FENSA altogether and needs a Building Control application regardless. Our FENSA guide, Part L guide and guide to fire escape windows go deeper; the requirements that bite on a fit are these.
| Requirement | What it means on the day | Source |
|---|---|---|
| Part L, thermal | WER band B or better, or whole-window U-value 1.4 W/m²K or less, and no worse than the old window. Units draught-proofed. | FENSA compliance guide v2023 |
| Part F, ventilation | Trickle vents: same equivalent area as before, or 8,000 mm² per habitable room and kitchen, 4,000 mm² per bathroom, if the old window had none. At least 1.7 m above the floor. Night-vent positions do not count. | FENSA compliance guide v2023 |
| Part K, safety glass | Toughened or laminated glass below 800 mm from the floor, and within 300 mm of a door below 1,500 mm. The mark must stay visible after fitting. Restrictors to 100 mm on low openers above a 600 mm drop. | FENSA compliance guide v2023 |
| Part B, escape | Fire escape windows: 0.33 m² clear opening, at least 450 mm each way (734 mm the other way if one side is 450), bottom of the openable area no more than 1,100 mm up. Make no worse than the old window, though an opening bigger than the minimum may be reduced to it; photograph it before removal. | FENSA compliance guide v2023; Approved Document B 2.10 and 2.19 |
| Part A, structure | A lintel or other adequate support above the opening. Replacement frames carry no load, and a soldier course is not support. Props on bays during the work. | FENSA compliance guide v2023 |
Source: FENSA Guide to Compliance, fourth edition (v2023.07). Scotland has its own building standards; see our Home Energy Scotland grant page for the Scottish route.
What fitting costs if you supply the window
The trade prices fitting per window. For a standard uPVC frame, labour only, the 2026 guides give £80 to £150, with removal and disposal of the old one at £25 to £30 and a FENSA certificate at around £25 where the fitter is registered. Fitters charge £25 to £35 an hour or £200 to £250 a day and usually work in pairs, so a team day is around £600, which does two to four windows. Several windows in one visit bring the per-window rate down, and upper floors may add a tower or scaffold at £40 to £150 a day. The full comparison of buying the frame yourself against a fitted price, including the guarantee you give up, is in supply-only vs fully fitted, and the should you fit your own windows tool puts your own numbers through it.
| Fitting-only cost | 2026 | Notes |
|---|---|---|
| Labour to fit a uPVC window | £80 to £150 per window | Checkatrade £80; MyBuilder £100 to £150 |
| Labour to fit a timber window | £80 to £100 per window | Checkatrade |
| Remove and dispose of the old window | £25 to £30 per window | Plus waste at around £150 per tonne |
| FENSA certificate via a registered fitter | about £25 per window | Against £220 to £370 for a DIY building notice |
| Fitter's rate | £25 to £35 per hour, £200 to £250 per day | Two-person team about £600 per day |
Source: Checkatrade window fitting cost guide (July 2026); MyBuilder (2026); Warwick Trade Windows (January 2026). Research in lib/research/window-removal-fitting-2026.md.
If you would rather someone with a registration number did it
The saving on a self-fit is the £80 to £150 a window of labour, set against the certificate, the guarantee and the lintel question. For a fitted price on your own openings, with removal, disposal and the FENSA certificate included, tell us what you have and Leads 2 Trade will arrange quotes from installers covering your postcode.
Frequently asked questions
Bed the sill on silicone with its end caps on, sit the frame on it and level it, plumb the jambs, then check the head and the diagonals. Pack behind every fixing point so the frame cannot bow, fix through the jambs with at least two fixings a side at no more than 600 mm centres and 150 to 200 mm from the corners, 40 mm into the masonry. Refit the glass on its packers toe and heel, clip the beads back, fill the gap with low-expansion foam and seal outside with low-modulus silicone, leaving the drainage slots clear.
Measure the structural opening in three places across and three down and use the smallest of each. For a white uPVC frame up to 3 m wide the industry good practice guide deducts 10 mm in total from the width and from the height, rising to 15 mm up to 4.5 m and 20 mm beyond; coloured uPVC, which expands more, takes 15, 22 and 28 mm. The deduction is total, not per side: it leaves about 5 mm each side for packing.
Legally, yes, but a full replacement is notifiable under the Building Regulations, so you must apply to your council's Building Control before you start, pay the fee, be inspected and get a completion certificate. A FENSA or Certass installer self-certifies instead. Without either, you have a problem when you sell. The physical job is within reach on the ground floor; the paperwork and the lintel question are what send most people to a fitter.
Each council sets its own fee. For 2026/27, a building notice for replacement windows in a house runs from about £220 to £370 depending on the authority, with some charging a base fee plus £20 per window. Retrospective regularisation, if you fit first and apply later, is higher, about £356 to £445. A FENSA-registered fitter's certificate is typically about £25 per window by comparison.
Fitting-only labour for a standard uPVC window is £80 to £150 in 2026, with £25 to £30 for removing and disposing of the old one and around £25 per window for a FENSA certificate if the fitter is registered. Fitters charge £25 to £35 an hour or £200 to £250 a day, usually as a two-person team, and most quote per window with a lower rate for several at once.
In most cases, yes. If the old window had vents, the new one must provide at least the same equivalent area. If it had none, a habitable room or kitchen needs 8,000 mm² of background ventilation and a bathroom 4,000 mm², measured per room rather than per window, positioned at least 1.7 m above the floor. A night-vent handle position does not count.
In England and Wales a replacement window must reach a Window Energy Rating of band B or better, or a whole-window U-value of no more than 1.4 W/m²K, and must be no worse than the window it replaces. A centre-pane figure of 1.2 is accepted only where the character of a listed or conservation-area building has to be kept.