How wide can a dormer be under Permitted Development?

Under Permitted Development rights (GPDO 2015, Class B), a rear dormer on a house must not extend beyond the plane of the existing roof slope facing the highway. The dormer must not be wider than 50% of the width of the original roof plane (measured from the outer edge of the eaves). It must be set back at least 200mm from the original eaves and set in from the party wall or hip by at least 200mm. Side-facing dormers require planning permission in most cases.

What types of dormer are there?

The main dormer types are: flat roof dormers (the most common for rear extensions, offering maximum internal space), pitched roof dormers (more traditional appearance with a small gable), hip roof dormers (similar to pitched but with hips returning to the main roof), and eyebrow dormers (curved, used on thatched or slate roofs). Flat roof dormers provide the most headroom and usable floor area for a given size.

Does a dormer need planning permission?

Rear dormers on houses often fall under Permitted Development rights, meaning no planning application is needed, provided they comply with the conditions in the GPDO (size limits, materials, no higher than the ridge, etc.). Front dormers, dormers on flats, and dormers in Conservation Areas, AONBs, or National Parks usually require full planning permission. All dormers require Building Regulations approval regardless of planning status.

What Building Regulations apply to dormers?

A dormer loft conversion must comply with: Part A (structure — the dormer frame, trimmed opening, and any steelwork), Part B (fire safety — fire doors, escape windows, smoke alarms), Part F (ventilation), Part K (stairs and guarding), Part L (thermal performance — windows must meet minimum U-values, currently 1.4 W/m²K), and Part P (electrical work). A structural engineer should design the opening and support.

What window U-value is required for a dormer?

Under Approved Document L (2021 edition), replacement windows and new windows in extensions/conversions must achieve a maximum U-value of 1.4 W/m²K. This typically means double-glazed windows with low-E glass and argon gas fill. Triple glazing achieves even better performance (typically 0.8-1.0 W/m²K) and may be worth considering for north-facing dormers.

Dormer Sizing Calculator — What Size Dormer Can I Build?

Calculate dormer window dimensions, structural opening width, cheek cladding area, roof area and window size. Includes planning constraint checks and Part L guidance.

Roof Pitch

Angle of existing roof slope

Available Roof Width

Width of roof plane where dormer will be built

Desired Dormer Width

External width of dormer face

Dormer Type

Flat roof dormers give the most internal space

Floor-to-Ceiling Target

Target internal height at dormer face (min 2.1m)

Cheek Cladding Price (optional)

Price per m² for cladding the dormer cheeks

How We Calculate This

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 + 200mm (100mm trimming each side)
Planning limit: Max dormer width = 50% of available roof width
Cheek depth: Front height ÷ tan(roof pitch) — how far the dormer projects
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 (must have minimum 1:40 fall for drainage)
Pitched roof: Two slopes at ~30°, area = 2 × slope length × depth
Hip roof: Similar to pitched but ~15% less due to hip triangles

Planning constraints (Permitted Development)

Dormer must not exceed 50% of the original roof plane width
Must be set back at least 200mm from the original eaves
Must be set in at least 200mm from party walls and hips
Must not extend above the ridge line
Materials should match existing roof as closely as practicable

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.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Dormer Sizing Calculator — What Size Dormer Can I Build?

How We Calculate This

Key calculations

Window sizing

Dormer roof types

Planning constraints (Permitted Development)

Standards referenced

Frequently Asked Questions

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