Dormer loft conversion planning permission in London: the limits that decide it

Do you need planning permission for a dormer loft conversion?

Often not, but the answer is a set of tests rather than a yes or no. Class B permits the enlargement of a dwellinghouse consisting of an addition or alteration to its roof, and a dormer is exactly that. If every limit and condition is met, no application is needed. If one is breached, the whole dormer needs permission, not just the offending part.

The Planning Portal says the same in plainer terms: converting the loft of a house is considered permitted development, subject to limits and conditions. It adds that if those limits and conditions are not met, or the right has been removed in the area, a householder application is required. The rest of this guide is the detail behind that sentence.

Two other approvals sit outside the planning answer. The Planning Portal says building regulations approval is required to convert a loft into a liveable space, and a dormer that cuts into a party wall can engage the Party Wall etc. Act 1996. Our guide to building regulations for loft conversions covers the first.

How is the 40 or 50 cubic metre limit counted?

The test compares the cubic content of the resulting roof space with that of the original roof space. Paragraph B.3 defines the resulting roof space as the roof space as enlarged, taking into account any enlargement, whether permitted by Class B or not. An earlier dormer, a previous loft conversion or a hip-to-gable change all use up the allowance.

Original means the house as it stood on 1 July 1948, or as built if it was built after that date. The 2019 MHCLG guidance gives the same rule and says any previous enlargement must be included. It does not say how volume is measured, internally or externally. We therefore treat volume as a measured-survey question, not something to estimate from a photograph.

A terrace house here means a dwellinghouse in a row of three or more used as single dwellings, with further tests on party walls. That includes an end-of-terrace house in such a row, which gets the 40 cubic metre limit. A semi-detached or detached house gets 50.

What is the 0.2 metre eaves rule for a dormer?

Condition B.2(b) says the eaves of the original roof must be maintained or reinstated, and that the edge of the enlargement closest to the eaves must, so far as practicable, be not less than 0.2 metres from the eaves, measured along the roof slope from the outside edge of the eaves. The MHCLG guidance says that means from the edge of the tiles or slates, with protruding guttering not counted.

The words so far as practicable matter. The guidance says the 0.2 metre set-back is required unless it can be shown that it is not possible for practical or structural reasons. Two cases are outside the rule altogether: a hip-to-gable enlargement, and an enlargement that joins the original roof to the roof of a rear or side extension, the L-shaped dormer or back addition. In the second case B.2(b)(ii) also lets the enlargement extend beyond the outside face of an original wall.

For every other dormer B.2(b)(ii) says no part may extend beyond the outside face of any external wall of the original house. That is why a rear dormer is normally set back from the wall below, not flush with it.

Do conservation areas, flats and Article 4 directions rule out a dormer?

Yes in each case, for the permitted development route. Paragraph B.1(f) removes Class B on article 2(3) land. Schedule 1 to the Order lists it as conservation areas, areas of outstanding natural beauty, specified areas under the Wildlife and Countryside Act 1981, the Broads, National Parks and World Heritage Sites. The MHCLG guidance says roof extensions on that land need an application.

Flats and maisonettes in a building containing flats have no Class B right. Article 2(1) defines a dwellinghouse, outside Part 3 and a few other cases, as not including a building containing one or more flats, or a flat in such a building. The guidance also says the rights do not apply to houses that are flats. Our guide to conservation areas and Article 4 directions explains how to find out where you are.

An Article 4 direction withdraws specified permitted development rights in a defined area. Planning practice guidance says it only means the development cannot be carried out under permitted development, so an application is needed. An immediate direction can take effect at once but must be confirmed by the council within 6 months. A council can be liable for limited compensation, and a direction cannot stop development already carried out or started. A planning condition on the original permission can also remove the right.

How do you prove a dormer is lawful before building?

A lawful development certificate is the voluntary route. Section 192 of the Town and Country Planning Act 1990 lets you apply to the council to ascertain whether proposed operations would be lawful. If the council is satisfied they would be lawful at the time of the application, it must issue a certificate, and otherwise it must refuse. Lawfulness is then conclusively presumed unless there is a material change in the relevant matters before the work starts.

Applying is optional: section 192 says a person who wishes to ascertain lawfulness may apply. For a dormer it is most useful when a buyer, a lender or a neighbour will later ask whether the work needed permission. It says nothing about building regulations, which is a separate regime. Our guide to lawful development certificates explains what evidence a council expects.

What about party walls?

Planning permission and party wall consent are separate statutory regimes. Section 2 of the Party Wall etc. Act 1996 gives a building owner rights that include cutting into a party structure and raising a party wall, and a party structure notice under section 3 is generally needed before they are used, unless the adjoining owners and occupiers consent in writing. Whether a dormer engages those rights depends on how the new structure is carried, for example on a steel bearing in the party wall. Ask a surveyor before the structural design is fixed.

What is the simplest way to settle dormer planning permission?

Check the land, the house type and the volume, in that order. Those three decide dormer loft conversion planning permission in most London cases before any drawing is made. If the answer is not clear, send us the address and a photograph of the rear roof. We check the constraints for free and tell you whether you need an application or a lawful development certificate. Our dormer loft conversion plans service prepares the drawings, and our guide to permitted development rights covers the wider rules.