Do you need
planning permission
for a loft conversion?

What you could build

On a typical Victorian or Edwardian terraced house, a full-width rear dormer transforms the loft from a low, dark crawl space into a proper room with full standing height across the entire floor plate. Add a hip-to-gable conversion on an end-of-terrace and you gain even more - the sloped side roof becomes a vertical wall, opening up the space that was otherwise lost to the angle of the hip.

On a semi-detached house, a rear dormer combined with a hip-to-gable on the side can create a master suite with room for a double bedroom, an en-suite shower room, and still have space left for a walk-in wardrobe or a study nook. On a detached house with a larger roof, the possibilities are bigger again - two bedrooms and a bathroom are common, and some conversions create an entire self-contained floor.

All of this is possible without a planning application. The question is whether your specific project meets the conditions.

01The volume limit

Class B, Part 1, Schedule 2, GPDO 2015 — B.1(d)

The central rule is volume. Your loft conversion must not add more than a set number of cubic metres to the roof space. This is measured externally - the volume of the dormer as seen from outside, not the size of the room inside.

These limits are cumulative. If a previous owner added a dormer that used 25 cubic metres, you only have 25 remaining (or 15 on a terrace). The allowance does not reset when the house is sold.

02What you can do where on the roof

Class B, Part 1, Schedule 2, GPDO 2015 — B.1(b) to B.2(b)

Rear dormers

The most common type of loft conversion and usually permitted development. The dormer must sit below the ridge line of the existing roof, be set back at least 200mm from the original eaves, and not overhang the outer face of the original house wall. Within those constraints, you have significant flexibility on width and height.

Front dormers

Never permitted development. You cannot extend beyond the plane of any roof slope that forms the principal elevation and fronts a highway. A front dormer always requires a householder planning application (£548). For a detailed explanation, see our front dormer guide.

Hip-to-gable

Converting a hipped (sloped) side roof into a vertical gable wall is usually permitted development. This is common on semi-detached and end-of-terrace houses where the side roof slopes inward, reducing usable space. The volume it adds counts toward your 40 or 50 cubic metre allowance, so it's often combined with a rear dormer - which is why getting the volume calculation right matters.

Rooflights

Adding Velux-style windows to any roof slope - including the front - is usually permitted development under Class C. The rooflight must not project more than 150mm from the existing roof plane and must not exceed the highest point of the roof. Rooflights don't count toward the volume limit because they don't enlarge the roof space. On a simple conversion where existing head height is adequate, rooflights alone can transform a loft without any structural extension at all.

Mansard conversions

A mansard dramatically alters the entire roof structure - typically creating a near-vertical rear wall with a flat top. Because it fundamentally changes the shape of the building, mansard conversions almost always need planning permission. They are common in London and can be approved, but you will need to apply.

03Other conditions

Class B, Part 1, Schedule 2, GPDO 2015 — B.1(e) to B.2(c)

Materials must be similar in appearance to the existing house. The flat roof of a dormer is an exception - felt, lead, or zinc are acceptable because they're not visually prominent.

Side-facing windows must be obscure-glazed and non-opening unless the opening parts are more than 1.7 metres above the floor. This is one of the most commonly overlooked conditions. If your loft room needs a clear side window for light, you'll either need to position it above 1.7 metres or apply for planning permission.

Height: no part of the conversion can exceed the height of the highest part of the existing roof. The ridge line is the benchmark.

No balconies, verandas, or raised platforms. A Juliet balcony (a railing with no floor) is normally fine because it has no external platform. A full balcony with a floor you can step onto is never permitted development. See our balcony guide.

04When you definitely need planning permission

Class B, Part 1, Schedule 2, GPDO 2015 — B.1(a) to B.1(h)

Flats and maisonettes. Permitted development rights under Class B only apply to houses. If you live in a flat, you need planning permission for any external alteration - and you'll also need the freeholder's consent. See our leasehold guide.

Conservation areas, National Parks, AONBs, World Heritage Sites, the Broads. On designated land, roof extensions are not permitted development at all. You need planning permission for any dormer or roof addition. Rooflights on the rear may still be permitted under Class C, but on designated land front-facing rooflights also need permission.

Listed buildings. You need both planning permission and listed building consent. See our listed buildings guide.

Article 4 directions. Some councils have removed specific PD rights in certain areas. This is most common in conservation areas but can apply anywhere. Check with your council.

Houses created through a change of use under Classes G, MA, N, P, PA, or Q of Part 3 of the GPDO - for example, offices or shops converted to houses. These properties don't have PD rights under Class B.

05Planning permission is not building regulations

Two separate systems - you almost certainly need building regulations

This catches people out every time. Planning permission and building regulations are completely separate. Even if your loft conversion is permitted development and doesn't need planning permission, you will need building regulations approval.

Building regulations cover whether your existing floor joists can support a habitable room (they usually can't without reinforcement), fire safety (a protected escape route, fire doors, and smoke alarms on every level), thermal insulation, sound insulation between the loft and rooms below, and the design of the staircase. Your builder or architect handles this, but it's your responsibility as the homeowner to make sure it happens.

06Lawful Development Certificate

Town and Country Planning Act 1990 — Section 192

If your loft conversion is permitted development, you don't legally need any approval to start. But we strongly recommend applying for a Lawful Development Certificate (LDC) - a formal confirmation from your council that the work is lawful. The current fee is £274 via planningportal.co.uk.

Why bother? Because when you sell, the buyer's solicitor will ask whether the loft conversion has planning permission or a certificate confirming it didn't need one. Without either, you face delays, reduced offers, or the buyer walking away entirely. An LDC costs a fraction of what that problem costs you later.