How Big Can a Loft Conversion Be Without Planning Permission?

40 cubic metres on a terraced house, 50 on anything else. That is the amount by which the roof space may be enlarged under Class B without a planning application, and it is not an allowance per project. It is cumulative, measured against the roof the house was originally built with, and it counts everything a previous owner already added.

Nothing in it limits how wide a dormer may be, or how far it may project. A roof conversion is the only householder job on the site measured in volume rather than in metres, and that changes how you have to think about it.

Why volume, and what it does to the design

A rear extension has a rear wall to measure from. A dormer has nothing equivalent, so the order counts the space instead.

Because the limit is a volume, the trade-offs are yours to make. A full width dormer that stops well short of the ridge and a narrower one that runs right up to it can come out at the same figure, and either may be permitted. That is genuinely useful at design stage, and it is also why a dormer cannot be checked by eye or compared against the one four doors down.

The height cap sits alongside it and is a separate test: nothing may be higher than the highest part of the existing roof. On a shallow pitched roof that binds long before the volume does.

Terraced means something particular here

The 10 cubic metre difference between 40 and 50 turns on a definition, not on what the street looks like.

For these purposes a terrace house is one in a row of three or more dwellings, sharing a party wall with, or having a main wall adjoining, the dwelling on either side. An end of terrace, adjoining on one side only, is included. So the 40 cubic metre figure reaches a good deal of Colchester housing that nobody would describe as a terrace in conversation.

A semi detached pair is not a row of three, so a semi gets 50. It is worth being sure which you have before the arithmetic starts, because 10 cubic metres is roughly the difference between a dormer that works and one that has to be cut back.

Measured against the original roof, and cumulative

So a house with a modest 1990s dormer has already spent part of the allowance, and only the balance remains. The allowance belongs to the building rather than to whoever owns it, it does not reset on sale, and nobody is obliged to tell you at purchase. Our page on what the original house means in planning covers how to establish what yours was.

This is the single commonest reason a conversion that “definitely does not need permission” turns out to need one, and it is checked against the planning history for the address rather than by measuring the house as it stands.

What the numbers look like on a real roof

Two worked shapes, from assumed dimensions, to show where the allowance actually goes.

Take an interwar semi eight metres across the back with a roof pitched at about 40 degrees, which puts the ridge roughly 3.3 metres above the eaves. A flat roofed rear dormer 5.5 metres wide, with its face set about 2.8 metres in from the ridge horizontally, encloses a wedge of roughly 3.3 square metres in section. Multiply by the width and it comes to something in the region of 18 cubic metres. Comfortably inside 50, with room to spare.

Now square off the hip on the same house. Filling a hip adds far more than people expect, because the missing volume tapers over a run equal to half the span. On those dimensions it works out somewhere around 35 cubic metres, most of the allowance in one move. Do both, which is the standard prescription for a 1930s semi, and you are past 50 and into a planning application.

Those figures are arithmetic from assumed dimensions and not a statement about your roof. Pitches vary by several degrees across houses that look identical, and a couple of degrees moves the answer. The point is the proportion: a hip to gable is the expensive item in volume terms, and the dormer is usually the cheap one.

What does not come out of the allowance

Rooflights are the useful exception, and they are the reason a marginal roof can still gain light.

A rooflight sitting in the plane of the slope enlarges nothing. It sits in Class C, which governs alterations rather than enlargements, and it is tested on a 150mm projection rather than on volume. So rooflights do not consume the 40 or 50 cubic metres, and neither does re-covering the roof. Where the allowance is nearly spent, that is often the whole answer: light comes in through the slope instead of through a box.

The internal work does not count either, because it is not development. Boarding, insulating, partitioning and putting a staircase in enlarge no roof space at all. The distinction between the two roof classes is set out on whether a loft conversion is permitted development.

Finding out what has already been spent

Search the address on the council’s planning register for a previous application or a Lawful Development Certificate covering roof work. Compare the roof against an un-extended house of the same type in the same terrace or pair, which is the quickest visual check available. And read the conditions on the original permission if the house is estate built, because on a lot of housing from the 1980s onwards the rights were removed entirely and the volume question never arises.

Where the calculation is close, have it worked out properly on a drawing rather than estimated on site, and apply for a certificate. A dormer 300mm too wide is not a small problem if the whole conversion depended on being permitted development.

The practical answer

Fifty cubic metres on a semi or a detached house, forty on a terrace including an end of terrace, minus whatever the roof has already gained since it was built. On most houses the binding constraint is not that number at all: it is the ridge height, the head height under it, and whether the address has the rights in the first place.

These limits are amended from time to time and how they apply depends on your specific property and its planning history. Treat this as an explanation of how the allowance works rather than confirmation of what you can build.

Last reviewed August 2026.