Extending a house with an outbuilding already in the garden

If you already have a garage, a garden room or a shed in the garden, the outbuilding and extension curtilage coverage rules decide how much extension you have left. Permitted development for both householder extensions and outbuildings carries the same limit: buildings within the curtilage, other than the original house, must not cover more than 50 per cent of the curtilage once the footprint of the original house is taken out of the sum. Every structure you have already built comes off that allowance before the new extension is measured.

How the 50 per cent coverage test works

The condition appears twice in Part 1 of Schedule 2 to the General Permitted Development Order 2015. Class A, which covers extensions and alterations, and Class E, which covers outbuildings and other incidental buildings, each say the same thing in nearly the same words.

The sum works like this. Take the area of the curtilage. Subtract the ground area of the original dwellinghouse. That gives you the denominator. Then add up the ground area covered by every building, enclosure and container in the curtilage other than the original dwellinghouse. That is the numerator. If it exceeds half the denominator, the right is lost.

Note that the original house comes out of both sides of the calculation, and that extensions do not. An extension built five years ago is a building within the curtilage other than the original dwellinghouse, so it counts against you when you come to build an outbuilding, and vice versa.

What counts towards the coverage figure

Buildings, enclosures and containers. In practice that means:

• every extension added to the house since it was originally built
• detached garages, garden rooms, home offices, workshops and sheds
• greenhouses, summer houses and log stores
• swimming pools and other pools, which the Order treats as buildings for this purpose
• oil tanks, water tanks and similar containers
• walls and fences, which are enclosures, though the ground they cover is usually trivial

What does not count is just as useful to know. Hard surfaces are not buildings, so a driveway, a patio or a hardstanding adds nothing to the figure however large it is. Nor does a raised deck, on the usual reading, although a deck with an enclosed store underneath probably does.

Structures that have been demolished no longer cover any ground, so removing an old asbestos garage really does free up allowance. That is often the simplest way to make a scheme work.

The original curtilage and the 1948 date

"Original" is defined in article 2 of the Order. For a house built before 1 July 1948, the original dwellinghouse means the building as it existed on that date. For anything built since, it means the house as it was first built.

This matters on older properties. A Victorian terrace with a two-storey back addition that was there in 1948 has that addition counted as part of the original house, which both increases the exclusion and reduces the numerator. The same terrace with an addition built in 1970 has to count that addition against the allowance.

Evidence for the 1948 position is the practical difficulty. Ordnance Survey historic mapping, the council's own aerial photography, and deeds or old sale particulars are the usual sources. Where the record is thin, the council will take a view, and the view will be conservative.

The curtilage is a separate question again. It is the land around the house that serves it, which is often but not always the whole of the title. A paddock bought from a neighbour and used as grazing is unlikely to be curtilage. Including it in the denominator to make the sum work is the most common reason a certificate application is refused.

Buildings demolished and rebuilt

Replacing a garage on the same footprint is neutral for coverage. Replacing it with something larger uses up the difference.

There is a timing point worth understanding. The test looks at the position at the time the new development is carried out, so if you demolish the old building before building the new one, the old footprint is out of the sum. If you build first and demolish afterwards, there is a period during which both exist and the coverage is exceeded, which technically takes the new building outside the permitted development right.

That is not a theoretical risk. Where a certificate is sought later, the sequence is exactly what an enforcement officer or a buyer's solicitor will ask about, so demolish first and photograph the cleared ground with a dated image.

Extensions and outbuildings competing for allowance

On an average suburban plot the allowance is generous and nobody has to think about it. On a small terraced garden, or on a plot where a previous owner built a double garage and a large rear extension, it is the binding constraint.

Decide which you want more. A large garden office and a modest single-storey rear extension may both fit. The same office plus a wraparound extension may not, and at that point the choice is to reduce one, to demolish something, or to apply for planning permission for the whole of what you want. A householder application is not a failure. It removes the uncertainty and it allows a scheme that exceeds the permitted development limits in other ways too, which is often what the client actually wanted. The interaction with earlier work is covered further in the notes on extending a house that has already been extended.

Remember that Class E only covers buildings incidental to the enjoyment of the dwellinghouse. A garden building used as a separate living space, a business with visiting customers or an annexe is not incidental, regardless of coverage, and the boundary is discussed in the notes on where incidental use ends.

Measuring and recording the figures

Do it properly once and the file is good for years.

1. Establish the curtilage boundary and check it against the title plan and the use of the land.
2. Measure the ground area of the original dwellinghouse, using the 1948 footprint for older properties.
3. Measure the footprint of every existing building, enclosure, pool and tank.
4. Measure the proposed new footprint.
5. Calculate: total of all buildings other than the original house, divided by curtilage area minus original house area.

Measure externally, at ground level, including any part of the structure that covers ground. Use the gross external footprint rather than the internal floor area, because the test is about ground covered.

Presenting the calculation for a certificate

A lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful. For a coverage argument it is worth having, because the calculation depends on facts a future buyer cannot verify for themselves.

Submit a scaled site plan with each structure numbered and colour-coded, a table listing every structure with its measured footprint, the calculation shown as a fraction and a percentage, and the evidence for the 1948 or as-built position where that is in play. Add a short written statement explaining the assumptions, in the same way a Planning Statement sets out a policy case for a full application.

Officers deal with these quickly when the arithmetic is transparent and slowly when it is not. Show your working.

Browse all 66 PlanScope report types and start a draft if the scheme has moved beyond permitted development and needs a full application. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.