Always measure structural brickwork openings from the outside of the building in millimetres. Take three width measurements (top, middle, and bottom) and three height measurements (left, middle, and right). Always record the smallest width and smallest height as your reference dimensions, and measure both diagonals to verify opening squareness.

For standard uPVC and aluminium window frames, a standard fitting deduction of 10 mm is deducted from both the smallest width and smallest height (5 mm perimeter expansion gap on each side). For timber frames or larger multi-panel sliding doors, deductions may range from 10 mm to 15 mm to accommodate thermal expansion and perimeter sealant backing.

Measuring opposing corner-to-corner diagonals confirms whether the masonry opening is square. If the difference between the two diagonal measurements exceeds 5 mm, the aperture is out of square. Frame sizes must be reduced sufficiently or perimeter packers adjusted to ensure the replacement window installs completely level, plumb, and square.

Our professional surveyor fees are structured by travel time from our Croydon headquarters: Local tier (under 15 minutes) is £50 for up to 5 items, Extended tier (15 to 45 minutes) is £80 for up to 5 items, with additional units charged at £10 per item. Survey fees are payable upfront upon booking and are fully credited against your final labour installation invoice upon contract booking.

When measuring an aperture with an existing removable sub-cill, measure height from the underside of the existing cill to the brickwork head. If the existing stone or concrete cill is structural and permanent, measure from the top of the masonry sub-cill to the brickwork head and specify that no add-on external cill is required.