Cumulative outbuildings and your remaining allowance

Your remaining allowance for a garden building is whatever is left of the rule that no more than half your garden may be covered by buildings. The cumulative outbuildings curtilage allowance under Class E of the General Permitted Development Order counts everything within the curtilage other than the original house, and it counts structures put up by previous owners as well as yours. Work out the figure before you order a garden office, not after.

The number that matters is ground covered, measured as a footprint, against the area of the curtilage excluding the footprint of the original house.

Which structures count towards the limit

The test in Class E is that the total area of ground covered by buildings, enclosures, pools and containers within the curtilage, other than the original dwellinghouse, must not exceed 50 per cent of the curtilage area excluding the ground area of the original dwellinghouse.

So the list of things to measure is longer than most people expect:

• Sheds, garden offices, summerhouses, workshops and log stores.
• Detached and attached garages, including a garage built with the house if it was added later.
• Greenhouses, potting sheds and polytunnels in a domestic garden.
• Swimming pools, hot tub enclosures and pool plant housings.
• Oil tanks and other containers, subject to the separate capacity limit in the class.
• Any extension to the house built after the original house, because an extension is a building other than the original dwellinghouse.

That last one catches people out regularly. A large rear extension built in 2015 counts against the same 50 per cent as the shed you want to build now.

What is generally not counted: hard surfaces, which are dealt with under a separate class and carry the drainage expectation described in the note on hard surfaces under Class F, and boundary walls and fences, which fall under a different part of the Order. Pool and court enclosures are a different matter and do count. Where a structure is borderline, ask the council's duty planner rather than guessing, because the answer changes the arithmetic.

What "original dwellinghouse" means

The original dwellinghouse is the house as it was first built, or as it stood on 1 July 1948 for anything older.

For a 1930s semi that means the house without the 1970s kitchen extension, without the 1990s conservatory and without the garage added in 2004. All three of those count towards your 50 per cent, and the footprint you subtract from the curtilage is the smaller, original one.

This is why the calculation on an old house that has been extended several times often produces an uncomfortable answer. It is also why the planning history for the property is worth pulling before you measure anything.

Structures built by previous owners

The allowance attaches to the land, not to the person. A shed, garage or extension built in 1998 by someone you have never met still counts.

There is no reset on sale, no reset after a period of years, and no credit for structures you did not build. The only thing that removes a structure from the calculation is removing the structure.

For buyers, this is a question to ask before exchange. A plot with a double garage, a large extension and a pool house may have nothing left at all, and a buyer who intends to add a garden studio should know that before completing rather than afterwards.

Measuring footprint rather than floor area

Three measurement points decide the answer.

Measure ground covered, not internal floor area. A timber building with 100mm walls has a footprint slightly larger than its internal area, and on several buildings those differences add up.

Measure the external face at ground level. Roof overhangs and eaves projections are generally not treated as ground covered, but canopies supported on posts and attached lean-to stores are.

Define the curtilage carefully, and keep it honest. The curtilage is the land serving the house, which is usually the garden plus the drive and the area around the house. A paddock bought separately from a neighbouring field is often not curtilage even though it is now in the same ownership, and including it to improve the arithmetic is the quickest way to lose the argument later.

Freeing allowance by removing a building

Demolishing an outbuilding is not usually development, so removing a tired garage or a collapsing shed is a straightforward way to recover allowance.

Do it in the right order. Remove the building first, photograph the cleared ground with a date, and keep the skip or contractor invoice. Then build. A claim that the new building is lawful because an old one "was going to be removed" is not a claim, and an officer responding to a neighbour's complaint will look at what is on the ground on the day of the visit.

Where the plan is to replace like for like on the same footprint, say so on any drawings you prepare and keep the before and after measurements together. Replacement is also the most reliable route where the allowance is tight, because the arithmetic does not change.

Recording the calculation for a future buyer

Put the working on one sheet of paper and keep it with the house papers.

It needs: a plan of the curtilage with a scale and a north point, the curtilage area with the original house footprint identified and subtracted, a schedule of every existing building with its footprint and the date it was built, the total, the 50 per cent figure, and the remaining allowance. Add photographs and the planning history references.

That sheet does three things. It tells you whether the next project fits. It answers a conveyancer's enquiry in one document. And if the council ever asks, it shows the calculation was done rather than assumed.

For a building that matters commercially, for example where a mortgage or a sale depends on it, a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990 gives a formal confirmation that the building does not need permission. The determination period is eight weeks and the fee is lower than a full application.

When the allowance is already used up

Three options remain, and none of them is a disaster.

Apply for householder planning permission. On a modest garden building in an unremarkable location this is often straightforward, and the application needs little more than a site plan, floor plans, elevations and a short statement. A Planning Statement explaining the purpose, the scale and the effect on neighbours covers most of what an officer wants.

Replace rather than add, keeping the total unchanged.

Or reduce the footprint and build up to the height limits instead, remembering that Class E permits a single storey only, with a maximum of 4 metres to a dual-pitched roof, 3 metres otherwise, 2.5 metres where any part is within 2 metres of a boundary, and eaves no higher than 2.5 metres. Within the curtilage of a listed building the ground area of a new building is capped much lower, and in a conservation area or other designated land a building more than 20 metres from any wall of the house is not permitted at all.

Whatever the route, the building still has to be for a purpose incidental to the enjoyment of the house, which is a separate test from the area calculation and is explained in the note on incidental to enjoyment of a dwellinghouse. A garden building used as a separate dwelling or run as a business fails that test however small it is, and the base rules on the area calculation are set out in the note on outbuildings under Class E and the 50 per cent curtilage test.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that works are lawful.

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