Do you need planning permission for a loft conversion or dormer?

Development is not permitted by Class B if ... the cubic content of the resulting roof space would exceed the cubic content of the original roof space by more than (i) 40 cubic metres in the case of a terrace house, or (ii) 50 cubic metres in any other case

The limits

• Volume. "Resulting roof space" takes account of any enlargement to the original roof space, "whether permitted by this Class or not" (B.3). Cubic content is measured externally (article 2(1)). An end-of-terrace house is a terrace house for this purpose (paragraph I).
• Height and front. No part higher than the highest part of the existing roof, and nothing beyond the plane of a roof slope that forms the principal elevation and fronts a highway (B.1(b), B.1(c)).
• Exclusions. No veranda, balcony or raised platform, and no chimney, flue or soil and vent pipe under Class B (B.1(e)). Pipes and flues fall under Class G.
• Materials. Similar in appearance to the house (B.2(a)). The technical guidance accepts felt, lead or zinc on a dormer's flat roof.
• Eaves and set-back. The original eaves stay or are reinstated, and the dormer sits at least 0.2 metres from the eaves, measured along the slope, so far as practicable (B.2(b)(i)). Nothing extends beyond the outside face of an external wall of the original house (B.2(b)(ii)).
• Side windows. Obscure-glazed, and non-opening unless the opening part is more than 1.7 metres above the floor (B.2(c)).

A hip-to-gable enlargement is exempt from the eaves and set-back rules. An enlargement that joins the original roof to the roof of a rear or side extension, such as an L-shaped dormer over an outrigger, is exempt from all of B.2(b). Class B is not available to flats, or once a house has been extended upwards under Class AA (B.1(h)).

A worked example of the volume

A semi-detached house with a hipped roof. Measured externally, the roof is 7.2 metres front to back and the ridge is 3.0 metres above the eaves.

• Hip to gable. The new gable is a triangle 7.2 metres wide and 3.0 metres high: 10.8 square metres. With the hip at the same pitch as the main roof, it runs back 3.6 metres to the end of the ridge, so the added space is a pyramid: 10.8 × 3.6 ÷ 3 = 12.96 cubic metres.
• Rear dormer. 5.0 metres wide, with a face 2.4 metres high above the old slope. At this pitch the flat top meets the old slope 2.88 metres further back, so the added space is a wedge: 5.0 × 2.4 × 2.88 ÷ 2 = 17.28 cubic metres.
• Total. 30.24 cubic metres, inside the 50 cubic metre limit for a semi-detached house.

If a previous owner had already added 20 cubic metres to the roof, with or without planning permission, the total would be 50.24 cubic metres. The whole conversion would then need planning permission, although the new work adds only 30.24.

What this means for you

Find out what the roof looked like when the house was built, or on 1 July 1948 if older, and measure every change since. Put the figures into the permitted development checker. Building Regulations approval is required to turn a loft into living space (Planning Portal), and work to a party wall may need notice under the Party Wall etc. Act 1996 (do you need a party wall agreement).

Getting certainty

A volume calculation within a cubic metre or two of the limit is exactly what a lawful development certificate settles. Send the council the calculation with the drawings. From 1 April 2026 the fee for householder works is £274, and the council has 8 weeks to decide.