Loft conversion volume allowances and how to measure them

A loft conversion under permitted development is allowed to add 40 cubic metres of roof volume to a terrace house and 50 cubic metres to any other house. Those figures are cumulative against the original roof, they are measured externally, and they are consumed by any previous roof enlargement whether you carried it out or a previous owner did. Get the measurement wrong and the conversion is unlawful, so the calculation deserves as much care as the drawings.

The volume allowances for terraced and other houses

The right sits in Part 1 Class B of the Town and Country Planning (General Permitted Development) (England) Order 2015, which permits additions to or alterations of the roof of a dwellinghouse.

The definition of a terrace house in Part 1 is wider than people expect. It covers a house in a row of three or more buildings used as dwellinghouses where it shares a party wall with, or has a main wall adjoining, the building on either side, and an end of terrace house sharing on one side counts too. So an end of terrace gets 40 m³, not 50 m³.

Three other limits apply before the volume question arises. No part of the enlargement may exceed the height of the highest part of the existing roof. No part may extend beyond the plane of any existing roof slope that forms the principal elevation and fronts a highway, which is what rules out a front dormer on most streets. And Class B is not available at all on the land described in article 2(3) of the Order, which includes conservation areas, National Parks, National Landscapes and World Heritage Sites.

Class B also applies only to a dwellinghouse. A flat or maisonette has no equivalent right, as our note on extending a flat or maisonette and why it differs explains.

What counts towards the volume

The allowance covers the increase in the cubic content of the roof space, and the measurement is taken externally. That means the thickness of the roof structure and finishes counts, as well as the usable space inside.

So a dormer with a 300 mm insulated flat roof and 250 mm walls consumes more of the allowance than an internal measurement suggests, typically by a noticeable margin on a full-width rear dormer. Measure to the external faces.

Everything that enlarges the roof space counts. Rear and side dormers. A hip to gable enlargement, which is often the largest single consumer, because it fills a whole triangular volume. A roof raised at the ridge or the eaves within the height limit. And a new roof over a previously flat-roofed section where the space becomes part of the roof volume.

Rooflights and windows set within the roof plane do not enlarge the roof space and are dealt with separately under Class C, which permits other roof alterations provided they protrude no more than 150 mm beyond the plane of the slope and do not exceed the highest part of the roof. Class C remains available on article 2(3) land, which is why rooflights are often the only option in a conservation area.

Previous roof enlargements and the original roof

This is where most unlawful loft conversions come from. The allowance is measured against the original roof, and original means the roof as the house was built, or as it stood on 1 July 1948 for a house built before then.

Every enlargement since that date counts against the allowance, regardless of who carried it out, whether it had permission, or how long ago it happened. A 1990s hip to gable conversion by a previous owner may have used most of the 50 m³ before you started.

Establishing what the original roof looked like takes three sources. The deeds and any historical drawings held with them. Historical aerial imagery and street-level imagery, which show roof changes clearly. And the local authority's planning history for the address, which will record any application for a previous enlargement.

Where the history is unclear, measure the enlargement you propose and then measure any visible earlier enlargement conservatively. If the total is close to the limit, apply for planning permission rather than relying on the right. The margin for error is not worth the risk of an enforcement notice.

Measuring the enlargement in practice

The reliable method is to model both roofs and subtract.

1. Build the original roof geometry from the historical evidence, as a solid to external faces.
2. Build the proposed roof geometry as a solid to external faces.
3. Subtract the first from the second. The difference is the enlargement volume.
4. Add any earlier enlargement volume, measured the same way, to get the cumulative figure.

Doing this by hand is possible for a simple rear dormer, where the volume is close to a rectangular box with a sloped face, and errors creep in on hips, valleys and stepped roof lines. A three-dimensional model removes the argument, and any competent architectural technologist can produce one.

Round against yourself. A calculation showing 49.4 m³ on a 50 m³ allowance is a calculation nobody should build on.

The 20 centimetre eaves set back

Class B carries a condition that, 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 set back at least 20 centimetres from the eaves, measured along the roof slope from the outside edge of the eaves.

That is a design constraint with real effect. A rear dormer cannot run down to the gutter line, so the dormer face sits back and there is a strip of retained roof slope below it. On a shallow-pitched roof this consumes useful head height inside.

The words "so far as practicable" give a little room, and they are the reason some conversions with a smaller set back have been accepted. They are not a licence to ignore the condition. If your roof geometry really does prevent a 20 cm set back, document why before you build, because the burden of showing impracticability falls on you.

The condition does not apply to a hip to gable enlargement, which is one reason that form is popular despite consuming so much volume.

Materials and side facing windows

Two further conditions apply to every Class B enlargement.

Materials must be similar in appearance to those used in the exterior of the existing house. A grey fibre cement dormer cheek on a red brick and clay tile house will be argued about, and the safest approach on a visible elevation is to match the roof covering on the dormer cheeks and roof.

Any window in a wall or roof slope forming a side elevation must be obscure-glazed, and non-opening unless the openable parts are more than 1.7 metres above the floor of the room in which the window is installed. That applies to the window in the flank of a dormer as well as one in a side roof slope, and it is checked on site during enforcement investigations. Where the house is a period property, the choice of window pattern for the new opening interacts with the existing fenestration, which our note on replacement windows in an unlisted period house covers.

Building Regulations run alongside all of this and are not affected by the permitted development position. A new storey more than 4.5 metres above ground level generally needs a protected stair enclosure rather than escape windows, and that requirement often reshapes the internal layout.

Recording the calculation for the file

Whether you rely on permitted development or apply for permission, write the volume calculation down and keep it.

Put on the drawings: the original roof outline shown dashed on the proposed sections and elevations, the enlargement volume with the calculation method stated, any earlier enlargement identified with its volume and its evidence source, the 20 cm set back dimensioned, and the height relationship to the existing ridge.

Then consider a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990. It costs a fraction of what a retrospective application costs, it settles the volume question with the authority before you spend money, and a future buyer's solicitor will ask for it. Where an application is needed instead, a short Planning Statement setting out the design response usually suffices for a householder case, with a Heritage Statement added where the house is listed or in a conservation area.

This article is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

Enter your postcode on PlanScope to see which reports a loft conversion at your address is likely to need.