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What changes the cost of a new window sill?
A window sill quote changes with whether the sill is inside or outside, what it is made of, and whether the window itself is being replaced. This guide does not print a price, and a sill on its own is not the same job as a new window.
Last reviewed:Which sill are you talking about?
A window has more than one "sill", and the quotes diverge as soon as you mix them up.
- The internal board. The timber, MDF or tiled shelf on the room side. People put plants on it. It can rot where condensation runs, or where a leaking frame has dripped for years. Replacing that board, with the window left in place, is a joiner's or carpenter's job.
- The external sill. Stone, concrete, tile or timber outside, shaped to throw water away from the wall. If it is cracked or flat, water sits against the frame. That can be a mason's or a builder's job, and it may mean the frame has to be protected or released.
- A sill supplied with a new window. Many replacement windows include a new sill or a trim that covers the old one. That is a window replacement, not a board swapped on a Saturday.
A bay in Crewe, a cottage window in Nantwich or a flat window in Sandbach can have an internal board, an outside sill, or a sill that comes with a new window. Point at the sill itself, on a Cheshire window or any other. Anyone in the UK can post the job. This guide does not print a price for a board, a stone, or a window.
Resealing a window is the job when the frame stays and the complaint is a draught or a failed seal, not a rotten sill. Window replacement quotes is the wider window page if the whole unit is coming out. Use this page when the sill is the thing you can point at.
What the quote should itemise
| Line | What good writing says | What a thin quote skips |
|---|---|---|
| Which sill | Internal board, external weathering, or both | "Window sill" |
| Material | Timber, stone, tile, plastic trim, in their words | "To match" |
| Window unit | Stays, or is replaced. If replaced, the controlled-fitting rules [S1] | A new window hidden inside a sill price |
| Glass | Stays in, or a glazing unit is replaced. A replaced unit is on GOV.UK's tell-building-control list [S2] | Glass out "for access" with no paperwork |
| Rot | How far it goes, and what they will cut back to | Paint over soft wood |
| Water | Where they think the water came from | A new board on a wet wall |
| Making good | Plaster, reveals, decoration, outside render | A fitted board and a scarred wall |
| Scaffold or ladder | How they reach an upper window | Implied, and then extra |
| Waste | Old sill removed | Left in the garden |
| Scheme or building control | Named, if the window unit or the glass unit is replaced [S2] | Silence |
| VAT | In the total or not. Registration is at £90,000 taxable turnover [S4] | Added on the invoice |
When the window itself is part of the job
If the fitter has to take the frame out, stop calling it a sill. The Planning Portal says that since 1 April 2002 building regulations have applied to all replacement glazing, covering thermal performance and other areas such as safety, air supply, means of escape and ventilation [S1]. An external window is a controlled fitting, and the regulations set standards when it is replaced [S1].
When any window is replaced, the opening should be sized to provide at least the same potential for escape as the window it replaces [S1]. If the original window was larger than necessary for escape, the Planning Portal says the new opening can be reduced down to the minimum that page sets out [S1]. This guide does not print that minimum. Ask the fitter to say whether the opening stays the same size. This guide does not measure your window.
A registered competent person scheme installer can carry out the replacement without you involving local authority building control, and should give a certificate [S1]. Someone who is not registered needs the building control body to check the work [S1]. GOV.UK says that if the installer is on the scheme, you should receive a certificate of compliance within 30 days [S2]. FENSA says it monitors building regulation compliance for replacement windows and doors, and that a FENSA certificate comes with a FENSA installer [S5]. A sill-only repair may not be a FENSA job. Ask, rather than assuming the logo on a van covers a piece of timber.
GOV.UK sets out building regulations approval. GOV.UK: building regulations approval. QuoteZila is not affiliated with this organisation.
An internal board that is only a board
If the glass and the frame stay, and the soft timber is the shelf inside the room, say that in the post. A carpenter can cut out the board, deal with any wet plaster behind it, and fit a new one. Ask them what they will do if the wet patch is the frame, not the board. Our carpenters and joiners pages are that trade.
Do not fill a spongy board with filler and paint it. The softness is the fault. You need the fitter to find where the water is coming from.
Decoration around the reveal is often missing from the price. A new board that is proud of the plaster, or shy of it, needs making good. Put that in the quote. Ask whether the curtain rail, the radiator below the window and the tiles on a splashback stay put. A board that has to come out past a tiled reveal is a longer job than a board that lifts free, and the quote should say which one they have seen.
An external sill and the wall under it
Outside, the sill is there to push water clear of the wall. A cracked stone sill, a tile sill that has slipped, or a timber sill that has gone soft at the front edge is a weathering fault. A builder or a mason may be the right trade. Builders and glaziers both meet this opening. Say whether you want the stone repaired or the window replaced, so they do not both assume the other job.
Upper floors are above the ground. Ask how they will reach the sill. HSE says you are working at height if you work above ground or floor level, where a fall could cause personal injury if precautions were not taken [S6]. The quote should say what they will stand on. This guide does not describe a way to climb. HSE explains this in its work at height questions. HSE: work at height. QuoteZila is not affiliated with this organisation.
Planning permission for repainting a frame is the Planning Portal's minor-improvement example [S3]. Replacing an external sill on a listed building can be a significant work. The same page says listed building consent is needed for any significant works, internal or external [S3]. If the house is listed, or in a conservation area, ask the council before the stone is cut out. The page also says that in a designated area there may be extra restrictions, and that you should check with the local planning authority before carrying out work [S3].
How to compare the quotes
Citizens Advice says to try to get written quotes from at least three traders, that a quote is a fixed price and an estimate is a rough guess, and that some traders charge for quotes, so ask about this first [S7]. If the work will take a long time and a deposit cannot be avoided, push it down, do not agree to more than 25%, and get a receipt [S7].
Compare internal board with internal board, and window replacement with window replacement. The £90,000 figure is the VAT registration threshold for a business's taxable turnover, not a price for your sill [S4]. Ask if the price you are looking at includes VAT.
Jobs you might post
These are examples, not taken from real job posts.
- "Internal timber board under the kitchen window is soft at the front edge. The window stays. Please replace the board only, and say what you find in the plaster behind it."
- "Outside stone sill is cracked and water marks the brick. Please quote the sill, not a new window, unless you think the frame has to come out. If it has to come out, price that separately and say who tells building control."
- "Bay window boards, three of them, painted and swollen. Please say if this is still a joiner's repair or a glazier's replacement."
Posting the sill
A photo from inside and a photo from outside, taken from the ground, are enough. Up to five matched local businesses can send a quote [QZ2]. QuoteZila is free for customers and local businesses [QZ1].
Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.
Posting a job is not open yet
Questions about sill quotes
How much does a new window sill cost?
This guide does not print a figure. An internal timber board, a tiled outside sill and a stone sill that is part of a new window are different quotes. Ask which of those the price is for before you compare totals.
Is a sill the same as a new window?
No. The sill is the shelf or the weathering under the glass. The window is the frame and the glass. The Planning Portal's rules for a controlled fitting apply when an external window or door is replaced. They do not, on that page, describe swapping only the board inside the room. If the quote includes a new window, those rules apply. If it is only the sill, ask the fitter whether the controlled fitting is being replaced as well.
What has to be told to building control?
GOV.UK lists replacement of window and door units among work you need to tell a building control body about, unless you use an installer on a competent person scheme. It also lists most repairs and maintenance as examples of work you do not need to tell them about, except replacements of glazing units, which do need to be told. A rotten internal board is closer to a repair. A new glass unit is the exception they name. Ask which one your quote is.
Do I need planning permission?
The Planning Portal says you do not usually need to apply for planning permission for repairs, maintenance and minor improvements, such as repainting window and door frames. New windows of a similar appearance are on that list. A new bay window may need permission, and an upper-floor side window must be obscure-glazed and either non-opening or more than 1.7 metres above the floor level. A sill is not given its own line. Listed buildings need listed building consent for any significant works, internal or external.
The wood is soft. Is that only decoration?
Soft timber under the glass can be the internal board, the external sill, or both, and it can be a sign that water is getting in. Paint will not dry out rotten wood. Ask the quote to say how far the soft timber goes, and whether the glass or the frame has to come out to replace it. If the frame comes out, you are in the replacement-window conversation.
Does QuoteZila check window fitters?
No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.
This page is general information, not professional or legal advice. QuoteZila does not check Gas Safe, WaterSafe, NFRC, NICEIC or other registrations against the register on your behalf: look them up yourself.
Part of Cost guides. All services
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Sources
- QZ1: QuoteZila product fact. QuoteZila home and FAQ copy.
- QZ2: QuoteZila product fact. QuoteZila FAQ How does getting quotes work.
- S1: Building regulations: doors and windows, Planning Portal. Accessed 2026-10-29. https://www.planningportal.co.uk/permission/common-projects/doors-and-windows/building-regulations/
- S2: Building work, replacements and repairs to your home, GOV.UK. Accessed 2026-10-29. https://www.gov.uk/government/publications/building-work-replacements-and-repairs-to-your-home/building-work-replacements-and-repairs-to-your-home
- S3: Planning permission: doors and windows, Planning Portal. Accessed 2026-10-29. https://www.planningportal.co.uk/permission/common-projects/doors-and-windows/planning-permission
- S4: Register for VAT, GOV.UK. Accessed 2026-10-29. https://www.gov.uk/register-for-vat
- S5: FENSA, FENSA. Accessed 2026-10-29. https://www.fensa.org.uk/
- S6: Work at height: frequently asked questions, HSE. Accessed 2026-10-29. https://www.hse.gov.uk/work-at-height/faqs.htm
- S7: Before you get building work done, Citizens Advice. Accessed 2026-10-29. https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/before-you-get-building-work-done/