Planning & Permitted Development
Permitted Development rules for loft conversions
Loft conversions are permitted under two classes of the GPDO 2015. Class B covers anything that enlarges the roof — dormers and hip-to-gable — and carries volume, height, position and setback limits, and is withdrawn on designated land. Class C covers alterations that add no volume, such as rooflights, and applies almost everywhere.
Last reviewed
What Permitted Development actually is
Permitted Development is not an exemption from planning control. It is a grant of planning permission made in advance, nationally, by statutory instrument — the Town and Country Planning (General Permitted Development) (England) Order 2015. If your scheme meets every condition of a class, permission already exists and no application is needed. If it breaches one, you do not have a “slightly non-compliant” scheme; you have no permission at all.
Loft conversions are covered by two classes in Schedule 2, Part 1, and the first job on any project is working out which one you are in.
Class B — enlargement of the roof
Class B is the one that matters for a rear dormer or a hip-to-gable. It permits the enlargement of a dwellinghouse consisting of an addition to or alteration of its roof, subject to the limits in B.1 and the conditions in B.2.
B.1 — the limits (breach one and the class does not apply)
- B.1(a) and (b) — height. No part of the enlargement may exceed the height of the highest part of the existing roof, and it may not extend beyond the plane of the existing roof slope on the principal elevation. Measured to the outside of the finished roof, not to the ceiling.
- B.1(c) — highway frontage. No part may extend beyond the plane of any existing roof slope which fronts a highway.
- B.1(d) — volume. The cubic content of the resulting roof space must not exceed the original by more than 40 cubic metres for a terraced house or 50 cubic metres for any other dwellinghouse. Cumulative across all previous enlargements.
- B.1(e) — no balconies. The enlargement must not include a balcony, veranda or raised platform.
- B.1(f) — designated land. Class B does not apply at all on article 2(3) land: Conservation Areas, AONBs (National Landscapes), National Parks, the Broads and World Heritage Sites.
B.2 — the conditions (must be complied with as you build)
- B.2(a) — materials. Materials used in the exterior must be of a similar appearance to those used in the exterior of the existing house.
- B.2(b) — eaves setback. Other than in the case of a hip-to-gable enlargement, the edge of the enlargement closest to the eaves of the original roof must, so far as practicable, be not less than 200mm from the eaves, measured along the roof slope.
- B.2(c) — side windows. Any window in a wall or roof slope forming a side elevation must be obscure-glazed, and non-opening unless the opening part is more than 1.7m above the floor of the room it is in.
Class C — other alterations to the roof
Class C permits any other alteration to the roof of a dwellinghouse — the class a rooflight conversion is built under. It is a much shorter class, because it adds no volume:
- C.1(a) — the alteration must not protrude more than 150mm beyond the plane of the slope of the original roof when measured from the perpendicular with the external surface.
- C.1(b) — no part may be higher than the highest part of the original roof.
- C.2 — the same obscure-glazing and 1.7m conditions as B.2(c) apply to side-elevation windows.
Which class is your conversion in?
| Conversion | Class | Volume cost | Allowed on designated land? |
|---|---|---|---|
| Rooflight / Velux | C | None | Yes |
| Rear dormer | B | Typically 15–35m³ | No |
| Hip-to-gable | B | Typically 15–25m³ | No |
| Hip-to-gable + rear dormer | B | Often near or over the cap | No |
| Front dormer | Neither — needs planning permission | n/a | No |
When the rights have been taken away
PD rights are national, but they can be withdrawn locally in three ways, and none of them is visible from the street:
- An Article 4 Direction, which a council makes to remove specified rights across a named area. See GPDO 2015 — Article 4.
- A planning condition on the original permission — routine on modern estates, and it binds every subsequent owner.
- Listed building status, which does not remove Part 1 rights as such but makes them practically unusable without listed building consent.
Common questions
- What is the difference between Class B and Class C?
- Class B covers enlargements of the roof — anything that adds volume, such as a rear dormer or a hip-to-gable. Class C covers other alterations to the roof that add no volume, principally rooflights. Class B is subject to volume limits and is withdrawn on designated land; Class C is subject to neither.
- What counts as the 'original' roof?
- The dwellinghouse as it was first built, or as it stood on 1 July 1948 if it is older than that. It is not the house as you bought it. Roof enlargements carried out by any previous owner have already spent part of the allowance, and they count against yours.
- Can a dormer be built on the front of the house under Permitted Development?
- No. Class B.1(c) prevents any part of the enlargement extending beyond the plane of an existing roof slope that fronts a highway. In practice this rules out front dormers, and on a corner plot it can rule out a side dormer too, since a highway includes a public road, footpath or bridleway.
Sources
- GPDO 2015 — Schedule 2, Part 1, Class B — the roof-enlargement rules, including the 40m³/50m³ volume allowance
- GPDO 2015 — Schedule 2, Part 1, Class C — rooflights and other roof alterations that add no volume
- GPDO 2015 — Article 4 — how a council withdraws permitted development rights in a named area
- Permitted development rights for householders: technical guidance — the government's plain-English reading of Class B, with diagrams
Check your own roof, free
Answer a few questions about your property and we will work through every Permitted Development condition against your actual dimensions.
Start the free assessmentFree to check. The £29 full report is fully credited against your full structural design.
Related guides
Do I need planning permission for a loft conversion?
Most loft conversions in England need no planning permission — they are Permitted Development. Here are the six conditions that decide it, and the cases that fail.
The 40m³ / 50m³ volume allowance, explained
How much roof volume Permitted Development lets you add, what 'original' means, why the allowance is cumulative, and how a rear dormer's volume is actually calculated.
Rear dormer loft conversion rules
The Permitted Development limits that shape a rear dormer: volume, ridge height, the 200mm eaves setback, highway frontage, matching materials and side windows.
Article 4 Directions and loft conversions
An Article 4 Direction lets a council withdraw Permitted Development rights across a named area. What it can remove, how to find yours, and how to read it.
Written by the LoftNow team from the legislation and Approved Documents cited above, and reviewed against the rules our assessment engine implements. It is general guidance for properties in England, not advice on your particular property — Permitted Development is self-assessed, and the definitive answer for a specific house comes from your local planning authority, normally via a Lawful Development Certificate.