Cost of Window Trim Installation

When windows are replaced or a new window opening is created, the trim work — covering the internal window board, reveal linings, and architrave — often forms a significant part of the finishing cost. The specification and quality of trim affects both appearance and draught-proofing, and costs can vary considerably depending on material choice, window size, and whether associated plasterwork is involved. Most homeowners encounter this expenditure as part of a broader window replacement, extension, or loft conversion project.

Key points

• Window trim in UK renovation typically refers to the internal window board (the sill shelf), the reveal linings, and the architrave — the moulded trim framing the window on the room side.
• Materials range from MDF and painted softwood (most common and affordable) to hardwood and stone, carrying very different supply costs and finishing requirements.
• Labour typically accounts for 50–70% of total cost on small-to-medium jobs, particularly where plasterwork to the reveals is also needed.
• External cill replacement — in uPVC, cast stone, or natural stone — is a separate item from internal trim and is usually quoted independently.
• VAT at 20% applies to labour and materials for most residential repair and improvement work; a reduced rate applies in limited circumstances under HMRC Notice 708.

What counts as window trim?

The phrase covers several distinct components. Knowing which elements your project involves helps you compare quotes accurately.

Internal window board (windowsill): the horizontal shelf at the base of the window opening on the room side. Commonly supplied in MDF, painted softwood, hardwood, or stone. Fitted on top of the plaster or directly to the structural sill.

Reveal linings: the flat surfaces lining the sides and top of the reveal — the wall depth between the window frame and the room face. In masonry properties, these are usually plastered. In timber-frame or stud-partition walls, they may be lined in MDF or moisture-resistant board.

Architrave: the moulded or flat trim covering the junction between the wall plaster and the window frame (or lining) on the room side. Available in a wide range of profiles, including ogee, torus, chamfered, and bespoke joinery.

External cill: the sloped external ledge at the base of the window opening. Constructed in uPVC, cast stone, natural stone, or brick-on-edge. This is an exterior element and is always quoted separately from internal trim.

Indicative costs: internal window trim

The following are indicative UK cost ranges, last reviewed 2026-05-19. Actual costs vary by region, material specification, site conditions, and contractor. Always obtain at least three quotes.

Indicative UK costs, last reviewed 2026-05-19. Figures exclude VAT. Costs vary by region, access difficulty, and whether associated plasterwork is required.

What drives the cost?

Material choice is the single largest variable. MDF architrave and a painted softwood window board represent the most economical specification; hardwood or stone trims can cost three to five times more for the same opening.

Associated plasterwork is often the hidden cost in trim projects. Where an old window has been replaced with a differently-proportioned frame, the reveal plasterwork needs cutting back and making good. A plasterer's day rate of approximately £180–£350 including materials (most UK regions, as of 2026-05-19) can equal or exceed the carpenter's cost on a single-window job.

Window size and complexity affects material quantities and fitting time. Bay windows, shaped heads, and deep reveals all add cost. A square bay window with three lights requires three sets of reveals and complex mitre joints at the corners.

Location and access: ground-floor installation is straightforward; first-floor work in a room with no scaffold access adds time. Wet rooms and kitchens usually need moisture-resistant board grades, which cost more than standard MDF.

Preparation and strip-out: if existing trim, architrave, or an old stone cill needs breaking out and disposing of, allow for additional labour time and waste removal charges.

Quote comparison template

When requesting quotes for window trim fitting, ask each contractor:

• What is included and excluded — board, architrave, reveal linings, plasterwork, priming, and painting?
• What are the exact material specifications — species, grade, profile, and supplier?
• Is preparation included — removing old trim, making good fixings, sealing cut edges before fitting?
• Is VAT included in the quoted figure, or will it be added on top?
• What happens if the reveal or structural sill is found to be in poor condition once old trim is removed?
• What are the lead time and likely duration on site?
• Is waste disposal included, or charged separately?

Trim specification and energy performance

Poorly fitted window boards and reveal linings can create cold bridges and reduce the draught-proofing effectiveness of a new window. Where a replacement window has been installed to meet Building Regulations Part L, ensure the internal trim is fitted with appropriate draught-sealing at the junction between the frame and lining. This matters most in solid-wall properties where the thermal gap at the reveal is narrower than in cavity-wall construction.

When to get professional help

Basic painted MDF or softwood trim is within the capability of a competent DIYer with the right tools. Engage a qualified carpenter for:

• Hardwood or stone boards requiring scribed joints or adhesive bedding.
• Deep reveals needing purpose-made lining sets.
• Bay windows requiring compound mitres at the corners.
• Work that follows structural alterations where building control sign-off is still live.

How Housey can help

Whether you need trim fitted as part of a new window installation or as standalone carpentry work, Housey connects you with experienced window and door installers who can quote for the complete job — frame, glass, and finish.

Frequently asked questions

Is window trim installation subject to building control?

Replacing or fitting internal trim on an existing window is not itself a notifiable building works item. However, if the trim work is part of a window replacement that includes a new glazed unit, the glazing element must be notified via FENSA, CERTASS, or a local authority building control notice.

Does window trim need replacing when I replace a window?

Not always, but often. A new frame may have slightly different dimensions from the old one, meaning the existing architrave no longer fits cleanly. In many window replacement quotes, basic making-good of trim is included; bespoke or hardwood trim is typically scoped and priced separately.

Can I use standard MDF for a window board in a bathroom?

Standard MDF is not moisture-resistant. In bathrooms and kitchens, specify moisture-resistant MDF (sometimes labelled MR-MDF) and seal all cut edges before fitting — especially the underside of the board. A painted hardwood or solid-timber board offers a more durable long-term alternative in wet rooms.

How long does window trim installation take?

A standard internal trim set for one window — board, reveals, and architrave — typically takes a carpenter 2–4 hours. If plasterwork to the reveal is also required, add half a day to a full day for a plasterer. A whole-house re-trim following multiple window replacements is usually priced as a day-rate job.

Sources and further reading

• GOV.UK: VAT on buildings and construction (Notice 708) — GOV.UK / HMRC
• FENSA: replacement windows and building regulations — FENSA
• Building Regulations Approved Document L: Conservation of fuel and power — GOV.UK / MHCLG
• Checkatrade: window sill replacement costs — Checkatrade (commercial source; cost figures indicative only)