Most British homes have non-standard window openings because buildings settle and original openings rarely match modern manufacturer standards exactly. The two main approaches: order standard-size windows that fit within the opening (using packing or trim to bridge any gap), or order custom-size windows made to fit the exact opening.
Standard sizes are faster to manufacture (typically 4–6 weeks) and slightly less expensive. Custom sizes take longer (typically 6–10 weeks) and cost more, but produce a cleaner fit with smaller gaps to seal.
For typical replacement projects, the installer determines whether standard or custom sizes are appropriate based on the actual opening dimensions and the available product range. Heritage replacements, unusual openings, and bay or shaped windows almost always require custom sizing. Standard windows work well where openings closely match standard dimensions and where slightly thicker trim is acceptable.
Window manufacturers produce standard sizes that align with common British opening dimensions, typical rafter spacings, and historical British window conventions. These standard sizes allow efficient manufacturing - frames, glazing units and hardware can be made at scale and held in stock for fast delivery.
Common standard sizes
These dimensions are the manufactured frame outside dimensions. The opening they fit into is slightly larger — typically 10–20mm in each direction to allow fitting tolerance.
Custom windows are made to the specific dimensions required by the homeowner's opening. The manufacturer cuts frame profiles to the exact length needed, sizes the glazing unit accordingly, and adjusts hardware positions for the specific window. Custom dimensions can be:
Heritage windows, bay windows, shaped windows and listed building replacements are almost always custom-sized. Each opening is unique and standard products rarely fit acceptably. The standard vs custom decision is largely irrelevant for these applications — custom is required.
The first question: how close are your actual opening dimensions to the nearest standard size? Within 10–20mm: standard often fits well. Within 50mm: standard with trim to bridge the gap is reasonable. Beyond 50mm: custom is usually cleaner and more cost-effective overall.
Custom windows typically cost 15–30% more than equivalent standard sizes. The cost premium covers:
The premium varies by manufacturer, material, and project scale. Whole-house custom orders attract smaller premiums than single-window custom orders.
For projects with deadline pressure (e.g. winter installation before serious cold), standard sizes' faster lead time can be valuable.
New-builds typically use standard openings designed around manufacturer-standard window sizes. Replacement windows fit naturally without custom sizing. Standard sizes are usually the right choice.
Properties from the 1960s onward generally have more regular openings than older properties. Where the existing windows are reasonably standard size and the openings are square, standard replacement windows work well.
Where the windows are less visible, the slightly larger trim associated with standard sizes is rarely noticeable. Standard sizes can be appropriate for less visible positions even when the homeowner has chosen custom for principal elevations.
Where budget is constrained, the 15–30% cost saving from standard sizes can be significant. Standard sizes deliver fully functional windows; the visual difference vs custom is rarely major.
Listed building consent typically requires the new windows to match the original size exactly. Original sizes rarely match modern manufacturer standards. Custom sizing is essentially required for heritage replacement work.
Victorian, Edwardian and earlier properties have window openings that follow conventions of their era, not modern manufacturer standards. Replacement with standard sizes often leaves visually awkward gaps. Custom sizing is usually preferable.
These windows are essentially custom in any case - each section of a bay or bow window is made to the specific angle and dimensions of the installation. The standard/custom decision doesn't really apply.
Older buildings have often settled over decades, leaving openings that are no longer square or that have moved from their original dimensions. Custom windows can accommodate the actual current shape of the opening; standard windows may not fit well.
Where the windows are highly visible - front of house, viewed from the street - the visual quality of custom-sized windows often justifies the additional cost. The fit, proportions and trim look better.
Where standard windows are used in openings that aren't a perfect match, trim bridges the gap between the frame and the opening. Different approaches achieve different visual results.
Well-executed trim is largely invisible. Plaster reveals painted the same colour as the surrounding wall look continuous; render beads disappear into the render. Trim becomes visible only when poorly executed or when the proportions are awkward.
Many projects use a mix: standard sizes where they work, custom where needed. This is often the best value approach.
This approach typically saves 10–20% vs all-custom while preserving most visual quality benefits on the windows where they matter most.
The pre-manufacture survey is where standard vs custom is determined. The installer measures the opening precisely and decides whether standard products can be used acceptably or whether custom is needed.
Homeowner input matters: communicate any visual quality priorities or budget constraints during the survey.
Even custom windows usually need minor fitting adjustments during installation. Packing strategies, sealant work, and trim positioning all happen on site. The custom dimensions are a starting point, not the entire installation.
Occasionally custom windows arrive with dimensions that don't match the actual opening - measurement error, manufacturing error, or building movement between measurement and installation.
The result is delays while the issue is resolved. Standard sizes have the advantage of being available faster if a replacement is needed.
Listed building consent typically requires new windows to match the originals in size. This usually means custom sizing because original windows rarely match modern manufacturer standards. The match needs to be precise — visible sightlines, glazing bar positions, sash horn details all need to align with the original.
Victorian sash windows often had specific traditional dimensions. Modern replacements need to follow these dimensions to look right — variation of 50mm in width can make a properly-detailed replacement window look out of place on an elevation of original sashes.
In multi-pane sash and casement windows, the position and width of the glazing bars affects the overall proportions. Custom manufacturing allows these details to match original; standard products may have glazing bars at slightly different positions or widths.
