Class B loft conversions: measuring the volume allowance
The allowance is 40 cubic metres for a terrace house and 50 for anything else, measured externally against the original roof. Earlier dormers count against it, whoever built them.
Most Class B loft conversion permitted development disputes come down to arithmetic that was done the wrong way round. The allowance is 40 cubic metres for a terrace house and 50 cubic metres for anything else, and those figures sound generous until you discover that the measurement is taken externally, that a previous owner's dormer eats into your allowance, and that a hip-to-gable enlargement uses a surprising amount of it before you have added a single dormer.
Class B sits in Part 1 of the General Permitted Development Order 2015 and covers enlargement of a house by an addition or alteration to its roof. It is the right that makes most loft conversions possible without an application, and it is also the right that generates the most refused lawful development certificates.
What Class B covers, and what it rules out
The right allows a roof enlargement: dormers, hip-to-gable conversions, and the joining of a roof to the roof of a rear or side extension.
It is unavailable in several situations, and the exclusions are absolute rather than matters of judgement.
- On land in a conservation area, National Park, National Landscape, the Broads or a World Heritage Site. Class B simply does not apply there.
- Where any part of the house would end up higher than the highest part of the existing roof.
- Where any part would extend beyond the plane of a roof slope that forms the principal elevation and fronts a highway. In plain terms: no dormer on the front roof slope facing the road.
- Where a verandah, balcony or raised platform is included. A roof terrace is out; a Juliet balcony with no platform is generally accepted.
- Where a chimney, flue or soil and vent pipe is installed, altered or replaced as part of the works.
- Where the house exists only because of a prior approval conversion under Part 3, such as an office or agricultural building converted to a dwelling.
Flats and maisonettes have no Class B rights at all, and an Article 4 direction can remove the right in a designated area. Check both before drawing anything.
For alterations that stay within the roof plane, such as rooflights, the relevant right is Class C rather than Class B, and the boundary between them is explained in Class C rooflights and the 150 millimetre rule.
Getting the cubic content right
Here is the rule that catches people: the cubic content of the resulting roof space must not exceed the cubic content of the original roof space by more than the allowance.
Two words in that sentence do the damage.
"Original" means the house as it was built, or as it stood on 1 July 1948 if it is older. It does not mean the house as you bought it. If a previous owner added a 20 cubic metre rear dormer in 1995, you have 20 cubic metres left on a terrace house, not 40. This applies whether or not the earlier work was lawful and whether or not you knew about it.
"Cubic content" is measured externally. You measure the volume enclosed by the outside faces of the walls and the outside of the roof covering, not the usable space inside. External measurement adds the thickness of the dormer cheeks, the roof build-up and the wall construction to your total, which on a wide box dormer can be several cubic metres more than an internal measurement suggests.
Work it as a subtraction rather than as a direct measurement of the new bits.
- Calculate the volume of the original roof space, from the top of the original eaves level to the ridge, using external dimensions.
- Calculate the volume of the roof space as it stands today, including any earlier dormers or enlargements.
- Calculate the volume of the roof space as proposed.
- Subtract the original figure from the proposed figure. That difference is what counts against the allowance.
- Show each of the three volumes separately, with the dimensions used, so a case officer can follow your working.
The volume below the original eaves level is not roof space and does not count. Neither does the roof over a rear extension, unless the proposal joins that roof to the original roof, in which case the newly enclosed space does count.
Hip-to-gable conversions consume volume quickly, because filling in the hip creates a wedge of new space before any dormer is added. On a modest semi that wedge can be 15 cubic metres or more, so calculate it before assuming a rear dormer will also fit.
Terrace, semi-detached or detached
The two-tier allowance produces more confusion than any other part of Class B, because the definition of a terrace house in the Order is not the one estate agents use.
| House type | Allowance | Point to watch |
|---|---|---|
| Terrace house, including end of terrace | 40 cubic metres | A row of three or more houses joined by party or adjoining main walls counts, and the end house in that row is still a terrace house |
| Semi-detached | 50 cubic metres | A pair of houses is not a row of three, so a semi gets the higher figure |
| Detached | 50 cubic metres | Straightforward, subject to everything else |
That middle row is the one people get wrong in both directions. A semi-detached house takes the 50 cubic metre allowance. An end-of-terrace house, which looks semi-detached from one side, takes 40.
Where the row includes a mix of houses and other buildings, or where an original terrace has been partly demolished, take advice. The detailed allowance tests are set out in Class B roof enlargement and the volume allowances.
The conditions: eaves, set back and side windows
Meeting the volume limit is necessary and not sufficient. Three conditions apply to the way the work is built.
Materials on any exterior work must be of similar appearance to those used on the existing house. Similar is not identical, and a tile that reads the same from the street usually passes, while a grey box dormer on a red brick terrace does not.
The eaves condition catches most designs. Other than for a hip-to-gable enlargement or one joining the original roof to a rear or side extension roof, the eaves of the original roof must be maintained or reinstated, and the edge of the enlargement nearest the eaves must be set back at least 0.2 metres from them, measured along the roof slope from the outside edge of the eaves. Draw that 200 millimetre strip on the section, because a dormer taken down to the gutter is the single most common failure.
Windows on a wall or roof slope forming a side elevation must be obscure-glazed, and must be non-opening unless the openable parts sit more than 1.7 metres above the floor of the room. The Order does not specify a level of obscuration, and councils commonly look for level 3 or above. Note that this is a condition on the development rather than a one-off requirement, so replacing that window with clear glass later takes the whole conversion outside Class B.
Recording the calculation
Permitted development is not a permission. It is the absence of a need for one, and the only formal way to confirm that works are lawful is a lawful development certificate under section 192 of the Town and Country Planning Act 1990 for proposed works, or section 191 for works already done.
For a loft conversion close to the limit, that certificate is worth the fee several times over. It reassures your building control body, it satisfies a future buyer's solicitor, and it stops an enforcement question arising in ten years when nobody can remember the dimensions.
Submit a proper pack: existing and proposed plans, elevations and sections at a stated scale, a volume calculation schedule showing all three figures and the dimensions behind them, photographs of the existing roof from front and rear, and evidence of the planning history including any earlier dormer. Where you are relying on the absence of earlier works, say so explicitly rather than leaving it to inference.
If the scheme exceeds the allowance, the alternatives are to reduce the dormer, apply for householder planning permission, or look at whether adding a storey under Class AA achieves the same accommodation under a different right.
PlanScope drafts a Planning Statement and supporting documents for householder applications, structured to UK planning policy and marked where measurements and site facts are still needed. Reports are AI-assisted drafts that should be reviewed before submission, and we do not guarantee planning approval. This is general guidance, not legal advice: permitted development depends on the facts of your house, so check the position with your local planning authority or a qualified professional before building.
Frequently asked questions
Is the loft conversion volume measured inside or outside?
Externally. You measure the volume enclosed by the outside faces of the walls and the outside of the roof covering, not the usable space inside. On a wide box dormer the thickness of the cheeks and the roof build-up can add several cubic metres compared with an internal measurement.
Does a previous owner's dormer count against my allowance?
Yes. The comparison is with the original roof space, meaning the house as built or as it stood on 1 July 1948 if older, not as you bought it. An earlier 20 cubic metre dormer on a terrace house leaves you 20 cubic metres, whether or not that earlier work was lawful.
Does a semi-detached house get 40 or 50 cubic metres?
Fifty. A terrace house in the Order means a house in a row of three or more joined by party or adjoining main walls, and a pair of semis is not a row of three. An end-of-terrace house, which looks semi-detached from one side, is still a terrace house and takes 40.
Why must a dormer be set back from the eaves?
Class B requires the eaves of the original roof to be maintained or reinstated, and the edge of the enlargement nearest the eaves to be at least 0.2 metres from them, measured along the roof slope. This applies except for hip-to-gable work or joining the roof to a rear or side extension roof.
Do I need a certificate for a permitted development loft conversion?
It is not compulsory, but for a scheme near the limit a lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal confirmation that the works are lawful. It reassures building control and removes the question when you come to sell.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.