Loft Conversion Volume Calculator

Check your loft conversion against the 40 m³ and 50 m³ permitted development limits in England, with the working shown.

1. Your house

House type A terrace house is one of a row of 3 or more houses. End of terrace counts as a terrace.

Previous roof enlargements (m³)Any earlier addition to the original roof space, permitted or not. Enter 0 if none.

2. Hip to gable

Number of hipped ends you will turn into gables

Hip 1 width (m)Across the hipped end, eaves to eaves

Hip 1 height (m)Eaves level up to the ridge

Hip 1 depth (m)From the end wall in to where the hip meets the ridge

3. Flat roof dormers Add a flat roof dormer

Dormer 1 width (m)Side to side, outside faces

Dormer 1 height (m)Height of the front face above where it meets the roof slope

Dormer 1 depth (m)From the front face back to where the flat roof meets the main roof

Add a second flat roof dormer

Any other roof addition (m³)For example a pitched roof dormer worked out by your designer. Enter 0 if none.

4. Quick rule checks The house is in a conservation area, National Park, Area of Outstanding Natural Beauty, the Broads or a World Heritage Site Part of the new roof will be higher than the highest part of the existing roof Part of it will extend beyond the front roof slope that faces a road (for example a front dormer) It includes a balcony, verandah or raised platform Part of it will extend beyond the outside face of an external wall of the original house (not joining a rear or side extension) Permitted development rights have been removed (Article 4 direction or a planning condition) The house was created by a change of use under permitted development, built under the new dwellinghouses class (Part 20), or has had extra storeys added under Class AA

Loft conversion volume calculator: how much roof space can you add?

In England, a loft conversion can usually go ahead without planning permission if the extra roof volume is no more than 40 m³ for a terraced house or 50 m³ for a detached or semi-detached house, and the other permitted development rules are met. Any earlier roof extension counts towards that allowance. This calculator adds up the volume of your hip to gable and dormers, compares it with your allowance and flags the other rules that would mean you need planning permission.

How to use this calculator

Pick your house type. Mid-terrace and end of terrace houses get 40 m³; detached and semi-detached houses get 50 m³.
Enter any earlier roof enlargement in cubic metres, even if it was done years ago or had planning permission.
Enter the dimensions of each hipped end you will change to a gable and each flat roof dormer, using measurements taken from the outside of the building.
Tick any rule check that applies, then press Check my loft volume. The result shows your total, what is left of your allowance and how each figure was worked out.

The rules explained

Loft conversions that add to the roof fall under Class B of Part 1, Schedule 2 of the Town and Country Planning (General Permitted Development) (England) Order 2015. In plain English:

Volume: the resulting roof space must not exceed the original roof space by more than 40 m³ for a terrace house or 50 m³ in any other case. "Resulting roof space" includes every enlargement, whether it was permitted development or not.
Original: means the house as it stood on 1 July 1948, or as first built if it was built after that date.
Height: no part can be higher than the highest part of the existing roof.
Front slopes: nothing can extend beyond the plane of an existing roof slope on the principal elevation that fronts a highway. That rules out most front dormers.
No balconies: verandahs, balconies and raised platforms are not allowed, and nor are new or altered chimneys, flues or soil and vent pipes under this class.
Designated land: Class B does not apply on article 2(3) land, which includes conservation areas, National Parks, Areas of Outstanding Natural Beauty, the Broads and World Heritage Sites.
Some homes are excluded: houses created by certain permitted development changes of use, houses built under Part 20 and houses already extended upwards under Class AA. Flats do not have these rights at all, so planning permission is needed.

Conditions you must also meet

Materials must look similar to the existing house.
Unless it is a hip to gable or joins the original roof to a rear or side extension, the eaves must be kept and the edge of the enlargement should, so far as practicable, be at least 0.2 m from the eaves, measured along the roof slope.
Unless it joins a rear or side extension, no part can extend beyond the outside face of an external wall of the original house.
Any window in a side wall or side roof slope must be obscure glazed and non-opening, unless the opening parts are more than 1.7 m above the floor of the room.

The government's permitted development technical guidance for householders explains these rules with diagrams, including the terrace house definition and how previous enlargements count. Councils can also remove these rights with an Article 4 direction or a planning condition. Rooflights that do not enlarge the roof are covered separately by Class C.

For more detail, read our guides to planning permission for a loft conversion, hip to gable loft conversions and loft conversion drawings. Planning a rear extension too? Try our permitted development rear extension calculator.

How the volumes are worked out

Flat roof dormer: treated as a wedge sitting on the roof slope. Volume = width × height × depth ÷ 2.
Hip to gable: filling in a hipped end turns a pyramid-shaped section of roof (one third of the box around it) into a gable section (one half of the box). The added volume is the difference: width × height × depth ÷ 6.

The Planning Portal volume calculators say all dimensions must be based on external measurements, not internal ones, and that standard shapes should be used with care when a roof is more complex.

Assumptions and limits

It covers England only. Wales, Scotland and Northern Ireland have their own permitted development rules, so this calculator does not apply there.
It assumes standard roof shapes with equal slopes. L-shaped roofs, mansards, pitched roof dormers and roofs of different pitches need a measured drawing, which you can enter in the "other roof addition" box.
Results are rounded to two decimal places. When you are close to the limit, small measuring differences matter.
Building Regulations approval is still needed for a loft conversion, whether or not planning permission is.
If your home is listed, speak to your council before planning any work.

Frequently asked questions

Is the 40 or 50 cubic metre limit measured inside or outside?

Outside. The Planning Portal states that all dimensions must be based on external measurements of the building, not internal measurements, so use measurements to the outside of the roof covering and walls.

Does a hip to gable conversion count towards the volume allowance?

Yes. The limit applies to the whole resulting roof space, so the volume added by a hip to gable and any dormers is added together. The 0.2 m eaves set back does not apply to the hip to gable part.

Do previous loft extensions count?

Yes. Any previous enlargement to the original roof space must be included, whether it was built under permitted development or with planning permission. "Original" means the house as it was on 1 July 1948, or as built if later.

Is an end of terrace house a terrace house for the 40 m³ limit?

Yes. A terrace house is one of a row of 3 or more houses, and the house at the end of the row counts as a terrace house, so the 40 m³ limit applies.

Can I build a front dormer without planning permission?

Usually not. No part of the enlargement can extend beyond the plane of a roof slope on the principal elevation that fronts a highway. Rooflights on the front slope may be possible under Class C if they project no more than 0.15 m.

Last checked 10th November 2026. This tool gives a guide only, based on the rules published at the date above. It is not professional advice, and your council has the final say.