Summary
Cill-only replacement is a job that gets underquoted more often than almost any other small domestic repair, because it looks simple from the outside — "just swap the cill" — but the actual work varies enormously depending on what's underneath and what's staying. This guide is scoped specifically to the standalone job: the window frame itself is structurally sound and is not being replaced, but the cill (sub-cill, external sill, or window board depending on region and trade) has rotted, cracked, spalled, or come away from its fixings and needs replacing on its own.
This is a genuinely different job from the cill make-good that's priced as an ancillary line item inside a full window replacement — where the frame is already coming out, the cill is accessible from above with no window in the way, and any masonry disturbance is already part of the wider job. Standalone cill replacement means working around a window that stays in place throughout, protecting the frame and glazing from damage during removal, and often dealing with masonry, pointing, or DPC detail below the cill that's been failing quietly for years before anyone noticed the wood rotting or the stone cracking.
The trigger for this job is usually one of three things: a timber cill has rotted from prolonged water ingress (often from a failed mastic seal or missing/damaged weathering groove), a UPVC cill has cracked or sagged from UV degradation or impact damage, or a stone/reconstituted stone cill has spalled from frost damage or a failed DPC allowing rising damp to track up through it. This guide covers material options, the removal and fitting sequence around a retained window, weathering and drainage detailing, and how to price the masonry unknowns that make this job risky to fixed-price.
Key Facts
- UPVC replacement cill (standard opening, up to ~1200mm) — £45–£90 material, £120–£220 labour (2–4 hours) — total £180–£320
- Hardwood/engineered timber replacement cill — £60–£140 material, £140–£260 labour — total £220–£420
- Precast concrete or reconstituted stone cill — £70–£180 material, £220–£420 labour (more masonry disturbance) — total £320–£600
- Natural stone cill (matched to period property) — £150–£450 material (bespoke cut), £250–£500 labour — total £420–£950; often bespoke-cut to match existing profile
- Bay window cill (3-section) — £500–£1,200+, driven by length, structural posts, and access
- Weathering groove/throating — a groove cut into the underside of the cill projection that breaks the water film and stops it tracking back to the wall face; must be present on any replacement cill, check it's specified on the order, not just assumed
- Minimum projection — cill should project a minimum of 25-40mm beyond the wall face (finished render/cladding) to throw water clear
- Fall/slope — external cills need a fall away from the frame, typically a minimum of 5-10° or equivalent slope, to shed water rather than pond against the frame
- DPC/DPC tray under the cill — a cill DPC or cavity tray beneath the new cill prevents water tracking down into the wall below; frequently missing or damaged on older properties and a common cause of the original failure
- Sealant — low-modulus, weatherproof silicone (not general-purpose) at the cill-to-frame junction; this joint fails faster than almost any other on the building envelope if the wrong sealant is used
- End caps/stop ends — UPVC and some timber cills need dedicated end caps where the cill meets brickwork/reveal at each end; a common item left off cheap quotes
- Cause diagnosis before quoting — establish why the original cill failed (missing weathering groove, failed sealant, DPC bridging, frost damage, physical impact) so the same failure isn't simply repeated on the replacement
- Access — ground floor cills are straightforward from a step or low tower; first-floor and above need proper access — see scaffolding pricing guide
- VAT — 20% standard rate; no reduced-rate applies to standalone cill repair on an existing dwelling
Quick Reference Table
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Try squote free →| Cill type | Material cost | Labour (hrs) | Typical total (single opening) | Notes |
|---|---|---|---|---|
| UPVC (standard opening) | £45–£90 | 2–4 | £180–£320 | Cheapest, fastest, good colour match to existing frame |
| Hardwood/engineered timber | £60–£140 | 3–5 | £220–£420 | Needs correct treatment class and weathering groove |
| Precast concrete/reconstituted stone | £70–£180 | 4–6 | £320–£600 | More masonry disturbance to remove and reset |
| Natural stone (bespoke matched) | £150–£450 | 4–8 | £420–£950 | Common on period/conservation properties |
| Bay window cill (3-section) | £250–£600 | 8–14 | £500–£1,200+ | Structural posts and length drive cost sharply up |
| DPC tray/cavity tray retrofit (if missing) | £25–£60 | +1–2 | Add £80–£180 | Frequently the actual root cause of the original failure |
| Weathering groove retrofit (if omitted on cheap replacement) | n/a | +0.5–1 | Add £30–£70 | Should be included as standard, not an extra |
Detailed Guidance
Diagnose before you quote
The first job on site is establishing why the existing cill failed — quoting a like-for-like replacement without fixing the underlying cause just repeats the failure in 5-10 years:
- Missing or worn weathering groove — a smooth-underside cill (common on older timber or poor-quality UPVC) lets water track back under the projection and drip against the wall face below, saturating masonry and undermining pointing over time.
- Failed or wrong sealant at the frame junction — general-purpose sealant hardens and cracks; water gets behind the cill and rots the timber or corrodes the fixings from underneath, invisible until the cill itself starts to fail structurally.
- Missing or bridged DPC/cavity tray beneath the cill — allows moisture to track down into the wall below the opening; this often shows as damp patches on the internal wall below the window, not just cill deterioration — see damp proof course pricing guide.
- Frost damage (stone/concrete cills) — repeated freeze-thaw cycles on a cill that's been saturated (usually because of one of the above causes) spalls the surface and eventually cracks the cill through.
- Physical damage — impact damage, or a UPVC cill that's sagged/cracked from age and UV exposure without any underlying moisture cause.
Flag the root cause to the customer in writing. If it's a DPC or sealant issue, fixing the cill alone without addressing the cause is a false economy that should be explained, not silently upsold.
Removing the cill without damaging the retained window
This is the defining difference from cill work priced inside a full window replacement, where the frame is coming out anyway:
- Protect the frame and glazing — mask and protect the retained window frame, especially if it's timber or a painted finish, before starting removal.
- Cut the sealant joint — score and remove the sealant bead between cill and frame cleanly; rushing this risks pulling paint or damaging the frame's underside.
- Release the cill fixings — screws, nails, or (on older properties) mortar bedding; on masonry-bedded stone or concrete cills this often means careful chiselling to avoid cracking adjacent brickwork.
- Check the sub-frame/opening below — once the old cill is out, inspect the masonry, DPC and any timber sub-cill or packing for rot or damage before fitting the new one. This is the point where "just swap the cill" jobs often reveal more extensive brickwork repair than anyone expected.
- Fit the DPC/cavity tray — replace or install if missing/damaged, before the new cill goes in — it cannot be retrofitted once the cill is bedded.
- Bed and fix the new cill — mortar bed for stone/concrete, mechanical fixing plus sealant for UPVC/timber, ensuring correct fall away from the frame.
- Seal the frame junction — low-modulus weatherproof silicone, tooled to a clean profile that sheds water rather than holding it.
- Make good — pointing, render, or plaster reveal make-good as needed above and around the new cill.
Material selection against the existing property
Match the replacement cill to what's staying, not just to cost:
- UPVC frames — UPVC cill is the natural match, widest choice of colour/profile, cheapest and fastest to fit.
- Timber frames (including sash windows) — hardwood or properly treated softwood cill matched to the existing profile; on period properties this often needs to be a bespoke-run profile rather than an off-the-shelf section — see sash window restoration pricing guide and heritage window repair pricing guide for the wider period-property context.
- Stone-built or rendered period properties — natural or reconstituted stone cill matched to the existing profile and stone type; conservation area properties may specifically require natural stone rather than reconstituted, which changes the price bracket significantly.
- Aluminium frames — aluminium or UPVC cill depending on the original spec; aluminium cills are less commonly replaced standalone since aluminium frame installations are newer on average.
Bay windows — the outlier that needs separate pricing
A bay window cill is not a scaled-up version of a standard opening — it's usually a run of three cill sections meeting at angled joints, sometimes sitting on structural posts or over a canopy roof below. Inspect the sub-cill structure and canopy (if present) before quoting; if the canopy roof (lead or tiled) below the bay has also deteriorated, that's a separate repair that should be priced and flagged, not absorbed into the cill quote. See the bay-window section of window replacement pricing guide for the wider structural considerations that apply equally here.
Weathering and drainage detail — where the job either lasts or fails again
Get these three details right on every cill, regardless of material:
- Weathering groove/throating on the underside — this is what actually stops water tracking back to the wall face; a flat-bottomed cill without one will eventually cause the exact same failure that triggered the replacement.
- Fall away from the frame — the cill must slope down and away from the window, not sit level or (worse) fall back toward the frame, which ponds water against the sealant joint and accelerates its failure.
- Adequate projection beyond the finished wall face — the cill needs to project clear of the render, cladding or brick face so water drips clear rather than running down the wall below the opening.
Frequently Asked Questions
How is this different from the cill replacement included in a full window replacement quote?
When a whole window is being replaced, the frame is already out, so the cill is accessible from above with nothing to protect, and any masonry disturbance below is already part of the wider job — that's why it's typically priced as a £150–£400 ancillary line item in window replacement pricing guide. Standalone cill replacement means the window stays in place throughout, so removal has to be done carefully around the retained frame, and the masonry/DPC condition below is often an unknown that hasn't been exposed by any other work. It generally takes longer per opening and carries more risk of hidden condition once the old cill is off.
Why did my cill rot/crack again after being replaced a few years ago?
Almost always one of: a missing or inadequate weathering groove letting water track back under the cill, the wrong sealant used at the frame junction (general-purpose rather than low-modulus weatherproof silicone), a missing DPC/cavity tray beneath the cill letting moisture track into the wall, or insufficient fall causing water to pond against the frame. A replacement that doesn't fix the original cause of failure will fail again on a similar timescale.
Can I replace just one section of a bay window cill?
Technically yes if the damage is localised to one section and the joints between sections are accessible, but matching the colour, profile and weathering of the existing sections precisely is difficult, especially on painted timber or older UPVC that's faded. Most customers end up preferring to replace the full run once they see how visible a partial patch would be, particularly on natural stone where exact colour matching is close to impossible.
Does replacing a cill need Building Control notification?
No — standalone cill replacement is a repair, not a controlled building operation, and doesn't trigger Part L, FENSA, or Building Control notification in the way a full window replacement does (see window replacement pricing guide for that separate compliance picture). If a DPC or cavity tray is being installed or repaired as part of the job, that work should still follow good practice per Approved Document C, but it isn't a notifiable event on its own.
Is a cracked stone cill always a sign of a bigger damp problem?
Not always, but it's common enough to check every time. Frost damage causing spalling is often the visible symptom of a cill that's been staying wet for longer than it should — which usually traces back to a missing weathering groove, failed sealant, or DPC issue rather than the stone simply being old. Inspect for damp staining on the internal wall below the window before assuming the cill crack is a standalone, cosmetic issue.
Regulations & Standards
Building Regulations Part C — site preparation and resistance to moisture; relevant to DPC/cavity tray detailing beneath the cill
BS 6262-4:2018 — glazing for buildings, code of practice for safety related to human impact (relevant where cill work disturbs glazing at low level)
BS 8000-3:2020 — workmanship on building sites, code of practice for masonry (relevant to stone/concrete cill bedding and pointing)
BS 5250:2021 — code of practice for control of condensation in buildings (relevant to moisture management at the opening)
Town and Country Planning (GPDO) 2015 — permitted development; relevant only where cill material/profile changes visibly alter a principal elevation in a conservation area or on a listed building
GOV.UK — Approved Document C (site preparation and resistance to moisture)
Glass and Glazing Federation guidance — industry technical documents on window detailing
Historic England — Traditional Windows — guidance on repairing period window components including cills
TRADA — Timber in external joinery — timber species and treatment guidance for external cills
window replacement pricing guide — full window replacement pricing, including cill make-good as an ancillary line item
sash window restoration pricing guide — period timber window repair, relevant where a bespoke cill profile is needed
heritage window repair pricing guide — conservation-area and listed-building window repair considerations
damp proof course pricing guide — DPC/cavity tray detail relevant when a missing DPC is the cause of cill failure
scaffolding pricing guide — access pricing for first-floor and upper cill replacement