Windows and Doors: Ordering, Lead Times, and Installation

Windows and doors are where your extension stops being a weatherproof shell and starts to feel like part of the house. Three things decide how well they perform: the frame material you choose, the glazing specification, and how carefully the gap around each frame is sealed. On top of that, they have to satisfy several parts of the building regulations at once: Part L for thermal performance, Part F for ventilation, Part K for safety glazing, and Part B for fire escape. Get those three decisions right and the rest is detail.

One of them needs making long before the rest, though, simply because of how long the windows take to arrive. Aluminium frames carry a 8-12 weeks lead time from order to delivery, stretching to 16 weeks in peak season, which makes them one of the longest-lead items on the whole build. Order them at foundations stage, not when the walls reach window height. Leave it too late and the building sits open to the weather while you wait: rain gets in, timber swells, plaster can't start, and your plasterer moves to another job.