Use an initial survey to identify constraints; obtain the supplier’s exact measuring instructions before ordering.
Record the opening
For an initial discussion, photograph the window straight on and from both sides. Record approximate reveal width and height at several positions, along with reveal depth. Traditional openings may taper or be out of square.
Label every window clearly. These are survey notes, not manufacturing sizes. The supplier must specify the measuring datum, fitting allowance and whether it needs tight opening dimensions or finished frame dimensions.
Check the obstructions
Record sill projections, handles, sash catches, shutters, curtain rails and blinds. Open the original window and check its full movement. A flush-looking inner frame can still obstruct a protruding catch.
Check the substrate around the opening. Loose plaster, unsuitable linings or fragile historic finishes may need a different fixing detail or preparatory repair.
Access, handling and safety
Ask about unit and panel weights, delivery access and the installation sequence. Large glazed panels require an appropriate handling plan. Check that the proposal preserves any required escape opening and ventilation.
Glass type and safety requirements should be confirmed for the location. Do not assume standard glass is suitable at low level, near doors or wherever impact or guarding requirements may apply.
Agree the finished result
Confirm frame colour, visible sightlines, trim details, seals, decoration and responsibility for making good. Ask who checks the final dimensions and who bears the risk if customer measurements are wrong.
At handover, operate every panel and the original window, inspect seals and finishes, and obtain cleaning, maintenance and warranty information. Keep the order specification with photographs of the finished installation.