How to Price Sash Window Restoration and Draught-Proofing

Summary

Traditional box-frame sliding sash windows are some of the most repairable joinery in the UK housing stock. A 150-year-old sash, properly restored, will outlast any modern replacement uPVC window — and on a Listed or Conservation property, restoration is usually the only legally permitted option. Yet customers are routinely sold replacement windows when restoration would have been the better answer technically, financially, and (on a Listed property) legally.

Pricing sash restoration accurately means understanding the scope continuum: from a quick re-cord and brush-pile (a half-day job, £350–£550) up to a full splice-repair-rebalance-reglaze with all four sashes apart on the bench (a 3–5 day job per window, £1,500–£2,500). The pricing variables are: number of splice repairs needed, glazing scope (retain original / slimline DGU / new single), paint condition (lead-paint test on pre-1960 properties), and Listed Building Consent constraints.

This article sets out the typical scope ladder, the per-window pricing, the regulation environment, and how to talk a customer out of replacement when restoration is the right answer. It is written for the carpenter, joiner, or specialist sash company quoting domestic work — not the surveyor or conservation officer specifying the work.

Key Facts

• Sash terminology — box frame (outer carcass), sashes (upper and lower glazed frames), stiles (vertical sides of sash), rails (top, bottom, meeting), parting bead (separates upper/lower sash), staff bead (holds lower sash in), pulley (top of stile box for cord), sash weights (in pockets in stile boxes)
• Draught strip systems — brush pile in carrier strip (Mighton, Reddiseals, Ventrolla) routed into parting bead and staff bead; provides air seal without preventing sash movement
• Re-cording — replacing failed sash cord (typically waxed cotton or polyester braided), 6mm or 8mm diameter, knotted to weight and tacked into sash stile
• Splice repair — cutting out rotten timber and scarfing in a new piece of matching timber (sash horn, sill nose, bottom rail, stile foot are common failure points)
• Sash weight rebalancing — total weight of both weights must equal sash weight (single-glazed sash typically 8–18kg; slimline DGU sash 12–24kg requiring heavier weights or supplementary lead)
• BS 644:2012 — Timber windows: factory-assembled windows of various types — specification (mostly applies to new manufacture, useful reference for restoration spec)
• BS EN 14351-1:2006+A2:2016 — Windows and doors — product standard, performance characteristics (CE/UKCA marking driver)
• Listed Building Consent (LBC) — required under Planning (Listed Buildings and Conservation Areas) Act 1990 for any work affecting character of a Listed building
• Conservation Area — Article 4 Direction may remove permitted-development rights for window replacement; restoration is the safe route
• Slimline DGU — slim double glazed unit (typically 11–14mm overall), fits into rebates that originally took single 4mm glass (with re-rebating)
• Crown glass / cylinder glass — original Georgian/Victorian glass with imperfections; high heritage value; reuse where possible
• Lead paint — pre-1960 paint layers may contain lead; test before sanding (HSE guidance L132 and PAS 4444 for paint testing)
• Part L1B — energy efficiency on existing dwellings; new windows U-value ≤1.4 W/m²K target; restoration with draught-stripping + slimline DGU typically achieves 1.5–1.9 W/m²K (acceptable on heritage properties via Historic England guidance)

Quick Reference Table

Detailed Guidance

Survey first — never quote off a photo

A sash window restoration quote off a photo is a quote that will be wrong. Survey on site. For each window:

1. Operate both sashes — does the cord pull? Does the sash run freely? Are there snags?
2. Check the timber: probe with an awl at the sill nose, bottom rail of lower sash, stile feet (where the sash meets the sill), and meeting rails. Soft = rot = splice.
3. Check the glass — single, slimline DGU, modern DGU. Note any original glass (imperfections, distortions).
4. Check the paint — bubbling, flaking, multiple layers. Pre-1960 = lead-paint risk.
5. Check the parting bead — split, damaged, painted shut?
6. Check the staff bead — pinned (easy to remove) or screw-fixed (modern)?
7. Check weight pocket access — can you access the pocket cover in the stile?
8. Check sill condition — front edge of sill is the most common rot point.
9. Listed status / Conservation Area — confirm before quoting any change of glazing.

Quote per window, not per house — every window is different.

Basic overhaul (the bread-and-butter job)

A typical re-cord + draught-strip overhaul:

1. Remove staff bead (preserve if intact — original profile is rarely available off-the-shelf).
2. Tilt out lower sash, cut old cords, lower weights inside the boxes.
3. Remove parting bead.
4. Tilt out upper sash, cut old cords.
5. Remove pocket covers, retrieve weights.
6. Ease and adjust sashes off the window — sand stiles smooth, plane where binding.
7. Replace any failed pulley wheels.
8. Rout in brush-pile draught-strip carriers (Mighton, Reddiseals, or similar) — typically into the parting bead, staff bead, and meeting rail.
9. Re-cord with waxed cotton or polyester — knot to weight, tack into sash stile cord groove.
10. Refit weights, hang sashes by replacement cord.
11. Test for balance — sash should hold at any position. If it drops, weights too light; if it rises, weights too heavy.
12. Refit parting bead and staff bead.
13. Re-putty any failed glazing bedding if doing glazing work.
14. Touch up paint where required (or full repaint quoted separately).

Time: 4–6 hours per window for a competent joiner with all parts to hand. Allow 8–10 hours for a first-time job.

Splice repairs

Rot is overwhelmingly found in:

• Sill nose — front edge of sill, exposed to driving rain
• Bottom rail of lower sash — sits on wet sill, soaks up water
• Stile feet of lower sash — meets sill, holds water
• Meeting rails — water runs from upper sash to meeting and pools
• Pocket cover area — paint failure here lets water track into stile box and rot the cill from inside

For a splice:

1. Cut back to sound timber with a 30° or 45° scarf joint (gives more glue surface).
2. Match timber species and grain orientation as closely as possible. Use durability-class 1 or 2 timber (Accoya, Sapele, Idigbo, Oak) — softwood splices in exposed locations will rot again.
3. Glue with PU adhesive (Sikaflex 11FC or similar — flexible) or epoxy resin (West System) for joinery-grade joints.
4. Pin and clamp until cured.
5. Plane flush, sand, prime with timber primer, undercoat, gloss.
6. For Listed work, document the splice with photos for the conservation officer.

A "minor splice" (sash horn, small sill repair) is 1–2 hours. A major splice (full sill nose replacement, bottom rail replacement) is 4–8 hours.

Slimline DGU upgrade — when it works and when it doesn't

Slimline DGUs (typically 11–14mm overall) can be retrofitted into traditional sash rebates with re-rebating. The benefit: U-value drops from ~5.0 W/m²K (single glass) to ~1.5–1.9 W/m²K (slimline DGU). Acoustic improvement is also material.

The constraints:

• Weight increase — slimline DGU is roughly 2.5–3× the weight of single glass. Sash weights must be uprated. Often requires supplementary lead in the pockets, or replacement of cast-iron weights with lead.
• Rebate depth — original rebate is 8–12mm for single glass; slimline DGU needs 14–18mm. Re-rebating reduces glazing bar / muntin section strength.
• Listed Building Consent — slimline DGU is sometimes refused on Grade I/II* Listed buildings because edge spacer is visible. Historic England's "Energy Efficiency and Historic Buildings" guidance (2018, updated) covers acceptable approaches.
• Bond failure risk — the slimmer the unit, the smaller the seal area; some units have a 5-year guarantee not the standard 10-year for modern DGUs. Specify a recognised manufacturer (Slimlite, Pilkington Spacia, FineO vacuum units, etc.).

For most Conservation Area properties (not Listed), slimline DGU + draught-strip + careful colour-matched painting achieves a workable energy compromise without removing the original sashes.

Listed Building Consent — get it before you quote

For a Grade I, II*, or II Listed building, work that affects character requires Listed Building Consent (LBC) from the local planning authority. This is not optional and is a criminal offence to do without (unlimited fine, up to 12 months imprisonment).

The trigger is "work that affects the character of the building as a building of special interest." Replacing single glass with slimline DGU usually triggers LBC. Re-cording and draught-stripping usually does not. Stripping paint to bare wood is sometimes argued either way.

For the quote: confirm with the customer they have LBC in place (or that the work is below the trigger). Get it in writing. Don't take on Listed work without conservation officer engagement on the spec — it's the customer's job to obtain consent but it's your job not to do unauthorised work.

For Conservation Area properties (non-Listed), Article 4 Directions in some councils remove permitted-development rights for window replacement, but restoration is generally unaffected. Check the local Article 4.

Lead paint on pre-1960 properties

Sash windows in properties pre-1960 are likely to have layers of lead-based paint somewhere in the stack. Dry sanding releases lead-laden dust which is a health hazard (especially for children in the property and for the operative).

• Test with a 3M LeadCheck swab (or send a sample for analysis) before sanding.
• If lead-positive: use wet sanding, chemical strippers (Peelaway 1 or similar), or controlled-removal methods.
• Wear FFP3 respirator, disposable coveralls, clean down before leaving site.
• Bag waste as hazardous if heavy contamination.
• HSE guidance: L132 "Control of lead at work" and HSE INDG305 "Lead and you."

This is a billable item — lead-paint procedures add £100–£250 per window to the quote.

Common pitfalls

• Quoting per house — every sash is different; quote per window.
• Skipping the splice diagnosis — quoting a £400 overhaul on a window that needs £300 of splice work means losing £300 on the job.
• Wrong cord — modern braided polyester is fine; sisal twine is not durable. Waxed cotton is traditional and lasts 20+ years.
• Weight mismatch after DGU upgrade — sashes that drop or jam ruin the job; rebalance properly.
• No paint protection on splice end-grain — end-grain wicks water; prime end-grain twice before assembly.
• Brush-pile too tight — sashes won't move; too loose, no draught seal. Test before finalising.
• Listed work without LBC — see above. Don't do it.

Pricing breakdown for a typical job

A typical "re-cord + brush-pile + 2 minor splice repairs + repaint outside, both sashes, single-glazed, original Victorian sash":

For a typical bay of three sashes, the per-window cost reduces with set-up efficiency: £1,650–£1,800 for the three.

Frequently Asked Questions

How long does a properly restored sash window last?

50–100+ years before the next major intervention. A correctly restored sash with periodic repainting (every 6–8 years on the exterior face) and re-cording every 25–35 years should outlast any modern replacement window. The Georgian and Victorian sashes still in use today (200+ years old) are the proof of concept.

Is restoration cheaper than replacement?

Per window, restoration ranges £350–£2,500; replacement uPVC or composite ranges £600–£1,800. Like-for-like timber sash replacement is £1,800–£3,500 per window. So restoration is usually cheaper than equivalent-quality timber replacement and within range of mid-spec uPVC. The longer-term cost picture favours restoration because the original frame box continues to perform — you're only replacing wear parts.

Can I get planning permission to replace sashes with uPVC?

In a Conservation Area with an Article 4 Direction, generally no — uPVC fails the character test. On a Listed building, definitely not. On an unprotected property in a non-Article-4 area, no consent needed. Many Conservation Areas without Article 4 still strongly discourage uPVC and a refusal at consent stage is common if you apply. Check with the local planning department before quoting replacement.

What U-value can a restored sash achieve?

Single-glazed restored sash with brush-pile draught-strip: 4.5–5.0 W/m²K (frame and glass). Add slimline DGU: 1.5–1.9 W/m²K. Add internal secondary glazing: 1.6–2.2 W/m²K. Modern uPVC DGU: 1.2–1.4 W/m²K. So restoration + slimline + (optional) secondary closes most of the gap without losing the original windows.

Are sash horns structural or decorative?

Functional. The horn is the bottom extension of the upper-sash stile below the meeting rail. It strengthens the tenon joint at the meeting rail (which would otherwise be a weak short-grain joint). Removing or simplifying horns weakens the sash mechanically and is a common giveaway of cheap restoration work. On heritage projects, reform horns to match the original profile.

Regulations & Standards

• Planning (Listed Buildings and Conservation Areas) Act 1990 — statutory framework for Listed Building Consent

• Town and Country Planning Act 1990 — Conservation Area framework, Article 4 Direction mechanism

• BS 644:2012 — Timber windows: specification (factory-assembled, useful reference for restoration matching)

• BS EN 14351-1:2006+A2:2016 — Windows and doors product standard

• Building Regulations Approved Document L1B — Energy efficiency on existing dwellings

• Building Regulations Approved Document F — Means of ventilation (trickle vent requirements on replacement windows)

• Building Regulations Approved Document K — Glazing in critical locations

• Building Regulations Approved Document Q — Security on new dwellings (less applicable to restoration)

• HSE L132 — Control of lead at work

• PAS 2030 / PAS 2035 — retrofit standards (relevant for funded energy work)

• Historic England: Energy Efficiency and Historic Buildings — Historic England technical guidance

• Approved Document L: Conservation of fuel and power — gov.uk

• Planning (Listed Buildings and Conservation Areas) Act 1990 — UK Government legislation

• HSE L132: Control of lead at work — Health and Safety Executive

• Society for the Protection of Ancient Buildings (SPAB) — windows technical guidance — SPAB

• British Woodworking Federation (BWF) — timber window technical guidance — BWF

• sash window restoration — technique deep-dive on restoration process

• draught proofing — draught-strip system selection and detailing

• upvc window repair — contrast pricing/methodology for uPVC repair

• window replacement pricing guide — replacement cost comparison

• glazing u values explained — U-value context for restoration vs replacement