Loft conversions: the permitted development limits

The limits

Working out the volume

The test compares the "resulting roof space" with the original roof space, and the resulting roof space takes in any earlier enlargement, whether or not it was permitted development (para B.3). So a dormer added by a previous owner uses up part of the allowance. "Cubic content" in the Order is measured externally (art 2(1)), and "original" means the roof as it stood on 1 July 1948, or as built if later.

For Part 1, a "terrace house" is a house in a row of 3 or more houses that shares a party wall or main wall with the house on either side, or, at the end of the row, with a house that does (para I). So an end of terrace house has the 40 m³ limit.

Conditions: materials, eaves and windows

• Exterior materials of a similar appearance to the existing house (B.2(a)).
• The eaves of the original roof must be kept or reinstated, and the edge of the enlargement nearest the eaves must, so far as practicable, be at least 0.2 m from the eaves, measured along the roof slope from the outside edge of the eaves (B.2(b)(i)). This does not apply to a hip-to-gable enlargement, or one that joins the original roof to the roof of a rear or side extension.
• No part of the enlargement may extend beyond the outside face of any external wall of the original house, except where it joins the roof of a rear or side extension (B.2(b)(ii)). Roof tiles, guttering, fascias, barge boards and other minor roof details that overhang the wall are not counted (B.4).
• Any window in a wall or roof slope forming a side elevation must be obscure-glazed, and non-opening unless the parts that open are more than 1.7 m above the floor of the room (B.2(c)).

Hip-to-gable conversions

Turning a sloping hip into a vertical gable is a Class B enlargement, so the volume, height and front-slope limits apply. It is exempt from the eaves and 0.2 m condition, but must not go beyond the outside face of the original walls (B.2(b)).

Rooflights: Class C

Rooflights that do not enlarge the roof fall under Class C. They must not protrude more than 0.15 m beyond the plane of the original roof slope, measured perpendicular to the roof, and must not be higher than the highest part of the original roof (C.1(b), (c)). A rooflight on a side roof slope must be obscure-glazed, and non-opening unless the parts that open are more than 1.7 m above the floor (C.2). Class C has no article 2(3) land exclusion.

Conservation areas and other article 2(3) land

Class B is not available at all on article 2(3) land (B.1(f)), which includes conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites (Sch 1 Pt 1). A dormer or hip-to-gable there needs a planning application, at a council fee of £548 for one house. See our conservation area check.

Class AA and other limits

Once a house has been built upwards under Class AA, Class B is not available (B.1(h)). The GOV.UK householder technical guidance was last updated on 10 September 2019 and predates Class AA, so it does not cover this; our check follows the Order as in force from 27 August 2026. Flats have no Part 1 rights (art 2(1)), and a planning condition or an Article 4 direction can remove Class B. A February 2024 government consultation proposed changes to householder rights, but GOV.UK still says "We are analysing your feedback", so the current limits stand.

What the free check tells you

The free check tests your plot outline against conservation areas and the other article 2(3) designations, so it shows whether Class B is ruled out on the data we hold, and lists the volume limit for your house type. It does not calculate roof volume: that needs drawings.

For certainty: a Lawful Development Certificate

For certainty, apply to your council for a Lawful Development Certificate for proposed works (TCPA 1990 s192). The council fee for a loft enlargement is £274, half the £548 application fee, and the decision is due within 8 weeks of a valid application. See Lawful Development Certificates.