The best time to install windows and doors in a new-build home is after the roof is on and the structure is weathertight, but before internal plastering and first-fix electrics and plumbing begin. The National House Building Council recommends this sequence to avoid damaging frames and to ensure proper sealing (NHBC Standards, 2026).
This timing applies to most new construction projects, including detached houses, extensions, and timber-frame builds. It does not apply to replacement installations in existing homes, where windows and doors can be fitted year-round. The key variable is the building stage: the shell must be secure enough to keep rain out, but the windows should go in before internal finishes that would be costly to repair if scratched or misaligned.
Install after the roof and walls are weathertight
Windows and doors should be fitted once the roof covering, felt, and battens are complete, and the external walls are built up to the correct openings. This allows the frames to be fixed securely into the structural openings without risk of water ingress damaging the wall cavities or insulation. The Building Regulations require that all new windows meet a minimum U-value of 1.6 W/m²K for the whole unit (Approved Document L, 2026). Installing them early ensures the airtightness layer is continuous around the frame, which is critical for energy performance.
Install before internal plastering and services
Once the windows and doors are in place, you can proceed with first-fix plumbing, wiring, and plasterboarding. Fitting windows earlier protects the interior from weather and allows plasterers to finish neatly around reveals. If you install after plastering, you risk damaging finished walls and creating gaps that compromise thermal performance. The Energy Saving Trust states that draught-proofing around windows can reduce heat loss by up to 20% (Energy Saving Trust, 2026). Proper installation timing directly affects this seal.
Install before external cladding or brickwork finishes
Window and door frames should be fixed before the final external finish, such as brickwork, render, or timber cladding, is applied to the outer leaf. This allows the frame to be properly integrated into the wall build-up, with correct cavity trays and flashings installed. The Fenestration Self-Assessment Scheme requires that all new windows and doors are installed in accordance with the manufacturer’s instructions and Building Regulations (FENSA, 2026). Fitting after the external finish creates water penetration risks and invalidates guarantees.
A worked example
A typical 1930s semi-detached house in Manchester adding a rear extension would see windows and doors installed after the roof is weathertight but before plastering begins, costing roughly £8,500 for three uPVC double-glazed windows and one composite back door. Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates that modern A-rated windows cut heat loss by up to 55% in an uninsulated solid-wall home, reducing annual heating bills by £195 for a gas-heated property. With the extension’s improved air tightness, the total yearly saving on the whole house reaches £260. The payback period sits at 33 years based on energy savings alone, but the added value to the property, typically 5% to 10% according to Energy Saving Trust, makes the investment worthwhile for comfort and resale.
| Item | Figure |
|---|---|
| Upfront cost after grants | £8,500 |
| Yearly savings | £260 |
| Payback period | 33 years |
| 25-year lifetime savings | £6,500 |
What homeowners often get wrong
The most common mistake is fitting windows and doors before the roof is fully weathertight, which voids warranties and leads to water damage in the wall cavities. Here are three frequent errors that cost time and money.
- Installing before the roof is complete Rain enters through unfinished roofs and soaks into timber frames and insulation, causing rot and mould within weeks. The right sequence is to wait until the roof covering, felt, and battens are all finished, then fit windows and doors to avoid voiding the manufacturer’s warranty.
- Skipping temporary weatherproofing Many homeowners assume new windows are immediately watertight, but the gap between frame and wall needs sealing with expanding foam and sill membranes. Without this, damp penetrates the cavity, leading to £1,500 in remedial plastering costs later.
- Ordering windows before the structural opening is measured Builders sometimes order windows based on architectural drawings alone, but brickwork tolerances can shift the opening by 10-20 mm. A frame that is too small requires costly packing and re-rendering, while an oversized frame means sending the order back with a 4-6 week delay.
Quick reference
- Install windows and doors after the roof is weathertight but before any internal plastering or first-fix electrics begin.
- The NHBC Standards 2026 recommend this sequence to avoid damaging frames and to ensure proper sealing against water ingress.
- All new windows must meet a minimum whole-unit U-value of 1.6 W/m²K under Building Regulations Approved Document L 2026.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- Installing windows too early risks voiding the manufacturer’s warranty if water damage occurs from an incomplete roof.
Frequently Asked Questions
After the roof is on and the structure is weathertight, but before internal plastering and first-fix services. This follows NHBC Standards 2026.
No, not recommended. The roof and walls must be weathertight to prevent water damage to frames and insulation, as per Building Regulations Approved Document L.
To protect the frames from plaster and debris, and to ensure a continuous airtightness seal around the frame for energy performance.
