Dormer Sizing Calculator — What Size Dormer Can I Build?

This calculator uses the roof pitch angle and desired dormer dimensions to calculate the structural opening, cheek sizes, roof area, and an appropriate window size.

Key calculations

• Structural opening width: Dormer width + a 200mm indicative trimming allowance (≈100mm each side). This is a setting-out rule of thumb only — the actual trimmed opening must come from the structural engineer’s Part A design, since trimmer/lining size varies with span and load.
• Permitted Development check: Added roof volume vs the GPDO Class B allowance — 40 m³ (terrace or end-of-terrace house) or 50 m³ (detached, semi-detached or any other house). There is no “50% of roof width” limit; that figure is a common myth, not statute.
• Cheek depth: Front height ÷ tan(roof pitch). This is an idealised upper bound that assumes the dormer face sits at eaves level; a dormer set back up the slope projects less, so treat it as a maximum.
• Cheek cladding area: 2 × (cheek depth × front height ÷ 2) — two triangular cheeks
• Flat roof area: Dormer width × cheek depth

Window sizing

The recommended window size allows for 150mm frame/reveal on each side and a 200mm head with 300mm cill height. For Part L compliance, windows should achieve a U-value of 1.4 W/m²K or better.

Dormer roof types

• Flat roof: Width × depth — design to a 1:40 fall so the 1:80 minimum finished fall is still achieved after construction tolerance and deflection (BS 6229:2018)
• Pitched roof: Two slopes at ~30°, area = 2 × slope length × depth
• Hip roof: Similar to pitched but ~15% less due to hip triangles

Permitted Development conditions (GPDO Class B)

• Added roof volume must not exceed 40 m³ (terrace or end-of-terrace house) or 50 m³ (detached, semi-detached or any other house) over the original roof — this is the only size limit; there is no 50%-of-roof-width rule
• No part may extend above the highest part of the existing roof (ridge)
• Must not extend beyond the plane of any existing roof slope that forms the principal elevation and fronts a highway (so most front dormers need planning permission)
• Set back at least 0.2 m (200mm) from the eaves, measured along the roof slope
• Materials of similar appearance to the existing roof
• Side-facing windows obscure-glazed and non-opening below 1.7 m above floor level
• A set-in from party walls or hips is sometimes requested by individual councils but is not a Class B condition

Standards referenced

GPDO 2015 Schedule 2 Part 1 Class B (Permitted Development), Approved Documents A, B, F, K, L, and P of the Building Regulations.