Every replacement window in the UK has to meet Building Regulations. Here's a plain-English summary of what those regs actually require.
Windows are governed by four Approved Documents: L (energy), K (safety in relation to impact), F (ventilation) and B (fire escape). Compliance is checked at the point of installation, either by Local Authority Building Control or by a Competent Person Scheme (FENSA or Certass). A good installer works to all four routinely.
Replacement windows must achieve a whole-window U-value of 1.4 W/m²K or better (as of Approved Document L 2022). Alternatively, a Window Energy Rating (WER) of C or above satisfies the same requirement. Modern double glazing with low-E and Argon fill meets this comfortably.
Any pane below 800mm from finished floor level must be safety glass — toughened or laminated. In doors and side-lights, this rises to 1500mm. Your installer will specify the glass to comply; check the survey drawings mark critical locations correctly.
New windows must maintain or improve the existing ventilation of the room. This usually means fitting trickle vents in the head of the frame — a small integral opening that provides low-rate background ventilation. Trickle vents are often overlooked on unbranded installations and become a snagging issue at sale.
At least one window in each habitable upper room must be an 'egress' window: minimum 450mm × 450mm openable area and minimum 0.33m² clear opening area. Your installer will identify which windows on the survey drawings need to meet this. Bathrooms and en-suites are exempt.
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