Windows for a new house or extension

Replacement windows in an existing home are usually signed off through a competent person scheme. That route does not apply here: FENSA, for example, states that it does not cover windows in new builds or new extensions. The windows are checked as part of the Building Regulations approval for the whole building, so the builder, the designer and the window company must work from the same specification.

That specification comes from the design stage. The energy assessment for a new dwelling, and the thermal calculations for an extension, assume particular window sizes and thermal performance. If cheaper windows with a worse rating are ordered later, the calculation no longer matches what was built, and sign-off can be delayed until it is redone.

Approved Document O on overheating applies to new residential buildings in England. Large glazed areas facing south or west, and windows that cannot be opened wide enough for night-time ventilation, are the usual reasons a design fails it. The window company needs to know the opening areas the designer relied on, and whether restrictors, shading or specific glass were assumed.

Approved Document Q deals with security. Requirement Q1 applies only to new dwellings and asks for doors and windows that can resist physical attack by a burglar, being strong enough and fitted with appropriate hardware. Ask the supplier to confirm that the windows and doors it offers meet the standard referred to in the approved document, and keep the evidence for Building Control.

Wales, Scotland and Northern Ireland have their own building regulations and standards, so a supplier working across borders should check which version applies to your site.

Windows are made to measure. Sizes are best confirmed on the built openings rather than from drawings, which means the supplier surveys once the walls are up, and lead times start from that point. On a timber frame or modular house, the openings are made in the factory and the windows can sometimes be fitted there too.

Windows often arrive when the building is not yet weathertight and other trades are still working. Agree who is responsible for protecting frames and glass from mortar, plaster and scaffolding damage, and who pays if a pane is cracked before handover.

How the window sits in the wall matters as much as the window itself. In a cavity wall, its position relative to the insulation affects cold bridging and the risk of condensation around the reveal. Cavity closers, damp-proof courses and cavity trays around the opening keep water out, and the gap between frame and wall should be sealed for airtightness, which is tested on new dwellings.

These details belong to the builder and the window installer together. Put the responsibilities in writing: who forms the openings, who fits the cavity closers and trays, and who seals the frame to the air barrier.

• window schedule matching the approved drawings and energy calculation
• thermal performance for each unit, and glass specification
• opening areas and restrictors assumed for overheating
• security standard for accessible windows and doors
• survey on built openings, lead time and delivery slot
• installation, sealing, protection and responsibility for damage

Describe your build, the number of openings and your timetable. Window companies in your area see that description without your contact details, and one of them can buy it. That company pays Offrano; you pay nothing. We send you its name, address and phone number before it can see who you are.

No company is obliged to take the request, which is withdrawn after 21 days without a buyer, and you remain free to decide. Offrano checks only that the buyer is a registered business. Ask for examples of new-build supply and for the evidence Building Control will need.