Replacement work is measured in the building that exists, not the building shown on an old plan. Walls move, reveals are rarely square and a previous frame may hide damage. A careful first survey does not replace a final site measure, but it makes that measure faster and exposes risks before a quote becomes a promise.
Record the opening in three dimensions
Measure width and height at more than one point, then record the smallest useful dimension and the variation. Check diagonals to understand whether the opening is out of square. Measure the reveal depth, sill projection, head condition and the distance to nearby corners or services. Photograph the tape in the opening so the number retains context.
Find the finished line
Decide which line the new frame must meet: structural opening, plaster finish, tile, cladding or the existing frame line. A replacement window can fit the structural hole and still clash with the finished reveal. Mark the internal and external finish separately. Note whether the sill slopes away from the building and whether a drip or flashing is present.
- Width at head, middle and sill
- Height at left, centre and right
- Both diagonals and visible twist
- Reveal depth and existing frame size
- Sill level, projection and drainage
- Access route, lifting and protection
Inspect what the old frame is hiding
Look for staining, soft timber, cracked render, failed sealant, corrosion and movement around corners. A moisture meter reading is useful only when you record where and when it was taken. Check whether the wall is cavity, solid, insulated or clad. The installation detail may need to change even when the replacement window looks similar to the old one.
Measure operation and hardware clearance
Record which sashes open, where handles sit and how far they travel. Check curtains, blinds, radiators, worktops and furniture. A new hinge or handle can need more clearance than the previous one. If the opening is near a path or boundary, note the required restrictor, safety glass or opening limit.
Separate survey facts from assumptions
Write “measured”, “observed” and “to confirm” beside each item. For example, the sill height may be measured, while the final frame size remains to confirm after the old unit is removed. This simple separation keeps a provisional quote honest and gives the installer a clear list for the final visit.
Questions to settle before fabrication
Should the new frame copy the old size?
Usually the existing frame is evidence, not a target. Confirm the structural opening, finish lines and installation tolerance before setting the final size.
What should be photographed?
Take each elevation, all corners, the sill and head, the room-side reveal, the delivery route and any services that could be damaged during removal.
Use the solutions page to compare opening approaches, then send the survey notes through contact before a final quote is prepared.
Leave a clear installation tolerance
Record the tolerance that the installer needs around the replacement frame and show where packers will sit. If the old frame is removed, allow for damaged masonry and a possible change in the finished line. A measured survey is valuable because it turns these unknowns into a short list that can be priced and checked.
Prepared by the joycham Windows + Doors editorial team. Specifications and performance outcomes vary by opening, glass, hardware and installation; confirm project-specific values before ordering.
