Entry 10 · Windows & Glazing
Measuring the opening: the survey behind a window quote
Understand what a window survey measures, which checks decide fit and ventilation, and what to ask before the figures become a firm quote today.
The survey is the visit that turns an estimate into an orderable unit, and what the surveyor records on it decides whether the new frames fit, ventilate and last. Expect the opening measured in more than one place, the squareness checked, the sill and reveal read for condition, and the ventilation and access noted, because a window made to a wrong figure is a problem no fitter can solve on the day.
The phrase the trade uses, free space, means the honest room left in the opening once the old frame is out, and the same two words mean something different on a home computer. Readers who enjoy that kind of precision can see how macOS counts its own headroom at Headroom, which explains measuring free space on a Mac in plain terms. In a wall, the same habit applies: measure what is actually there, not what the brochure assumes.
01 What does the surveyor actually measure?
A brick opening is never perfectly square, so the width and height are taken at several points and the diagonals checked for racking. The smallest reading governs, because the new unit must pass the tightest spot and the gap left at the edges is closed by packers and sealant rather than by forcing the frame. A survey that records only one width and one height is a survey you should ask to be done again.
Below and beside the opening, the surveyor reads what the new unit will sit on and against. The sill, the lintel line, the state of the reveal plaster and the condition of the brickwork all shape the fixing method and the making good afterward. Photographs of each elevation, taken now and kept with the order, give you a record if the state of the wall is later disputed.
02 Which checks decide the order?
Ventilation is the check most often missed. Background ventilation requirements apply to replacement windows in England, and a survey that records the room use, the existing vents and the heating pattern gives the order the right vent provision rather than a default. Structural signs matter too: cracking above the opening, a dropping lintel or damp at the reveal all change what the job includes and who should look first.
Access is the quiet third check. A bay over a conservatory roof, a window above a neighbour's extension or a third-floor opening all change the equipment the fitting day needs and the price it carries. Ask the surveyor to note access explicitly on the sheet so the quote you sign covers the day as it will actually run.
Your own measurements have a place in the file as well. They do not replace the survey, but a rough set taken with a tape before the visit lets you follow what is being recorded and spot a figure that strays. A quote you can read is a quote you can question.
03 How do you read the figures afterward?
The survey sheet should distinguish between the structural opening and the manufacturing size, and the difference between them is the fitting tolerance the installer has chosen. Ask which figure goes to the fabricator, what tolerance was allowed and who confirms the final measure if the wall turns out different once the old frame is out. Those three answers tell you whether the sheet is a record or a guess.
The survey profession itself has conventions for this kind of work, and the Royal Institution of Chartered Surveyors keeps the wider framework for how measurement of buildings is described. Your window survey is a simpler document than theirs, but the principle carries over: a measurement is a fact recorded against a stated method, and the method is what lets a later reader trust the figure.
04 Where do the numbers meet the contract?
The survey sheet belongs inside the contract, not beside it. When the order form lists the units, the sizes and the vent provision should trace back to the sheet, and any change agreed after the visit should be written down the same way. The ledger keeps a separate entry on reading the contract clause by clause, and the survey is the annex that gives those clauses their numbers.
Survey day is also the day to mention everything the figures cannot show: the noise that drove the order, the draught at the landing, the furniture that must move. The note on what new windows can and cannot stop shows why the acoustic side of the job belongs on the same sheet. A surveyor who writes it down is a surveyor whose measurements you can hold to.
Before the surveyor arrives, clear the sills, unlock the side access and write down the three things you want the visit to record. A survey that starts with your list ends with an order you can defend.
One figure on the survey sheet does more than size a window: the clear opening. In a bedroom, that opening is the escape route, and it must be wide enough and high enough for a person to pass through without removing the sash or any restrictor. The measurement is taken between the frame reveals, at the narrowest point, with the window fully open, and it is checked against the room's use. Anything fixed in the way, a blind, a handle, a locked restrictor or furniture, defeats the purpose. Households planning a bedroom escape should rehearse the opening before it is ever needed.
Once the survey figures are agreed, the sequence of work around the opening still has to be settled before the quote becomes a contract. Plaster repairs, sill and reveal finishes, and the point at which the old frame is removed all affect access and the final measurement. The Sash and Frame Ledger sets out the order of trades, dust control and the documents worth keeping in its entry on making good after fitting, which is a sensible next step once the survey is signed off.