Outbuildings under Class E and the fifty per cent test

Outbuilding permitted development curtilage rules come down to an area sum and a height rule. Under Class E of Part 1 of Schedule 2 to the General Permitted Development Order 2015, everything in the garden other than the original house, taken together, must not cover more than half the curtilage. And any outbuilding within two metres of a boundary is limited to 2.5 metres in overall height, which rules out most pitched roofs against a fence.

Those two constraints decide almost every garden room, home office, gym and store. The rest of Class E is a list of things that take you outside it altogether.

What Class E permits

Class E allows a building, enclosure, swimming pool or other pool within the curtilage of a house, for a purpose incidental to the enjoyment of the house, along with maintenance and alteration of such buildings.

The limits that take a proposal outside the class are:

• Ground cover above half the curtilage, excluding the original house.
• Any part forward of a wall forming the principal elevation of the original house.
• More than a single storey.
• Overall height above 4 metres with a dual pitched roof, 2.5 metres within 2 metres of a boundary, or 3 metres in any other case.
• Eaves height above 2.5 metres.
• A veranda, balcony or raised platform, a platform being anything more than 0.3 metres high.
• A separate dwelling, or a microwave antenna.
• A heating oil container over 3,500 litres.

On article 2(3) land, which means conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, two extra restrictions apply. Nothing may be built between a side elevation of the house and the curtilage boundary, and buildings more than 20 metres from any wall of the house may not exceed 10 square metres in total.

The word doing the most work is incidental. A building used for primary living accommodation, or occupied as a separate unit, falls outside Class E regardless of its size.

The fifty per cent curtilage calculation

The test is that the total ground area covered by buildings, enclosures and containers within the curtilage, other than the original dwellinghouse, must not exceed half of the total area of the curtilage excluding the ground area of the original dwellinghouse.

Read that twice, because the two exclusions work differently. The original house is excluded from the numerator, so its footprint does not count as built. It is also excluded from the denominator, so the area you halve is the curtilage minus the house footprint.

What counts in the numerator is wider than people expect. Every existing outbuilding, including sheds, greenhouses, a detached garage, a summerhouse, a log store and a fuel tank. Extensions to the house that were added after it was originally built, because "original" means as at 1 July 1948 or as first built. Hard enclosures. A swimming pool.

What does not count is uncovered hard surfacing, such as a patio or a driveway, which is controlled separately under a different class with its own drainage requirement.

A worked illustrative example. A plot measuring 420 square metres in total, with the original house covering 70 square metres. The curtilage for the calculation is 420 minus 70, which is 350. Half of that is 175. Existing structures comprise a garage of 15 square metres, a shed of 6 and a rear extension added in 1992 of 20, a total of 41. That leaves 134 square metres before the limit is reached. The figures are illustrative only.

Three cautions. The curtilage is the land used with the house, which is not always the same as the title boundary, and a paddock bought later may not be curtilage at all. Flats have no curtilage for these purposes, so Class E does not apply to them. And where the house was built after 1948 within a larger plot, the curtilage as originally provided is what counts.

Height limits and the two metre boundary rule

Height is measured from the highest ground level immediately adjacent to the building, which matters on a sloping garden where a building can be 2.4 metres on the uphill side and 3.1 metres on the downhill side.

Within 2 metres of any boundary of the curtilage, the maximum overall height is 2.5 metres. That is the constraint that reshapes most designs, because 2.5 metres with a usable internal height leaves almost nothing for a roof build-up. The usual responses are a flat or very shallow mono-pitch roof, a slightly sunken floor level, or moving the building 2 metres clear of every boundary.

Beyond 2 metres from every boundary, the limits are 4 metres overall for a dual pitched roof and 3 metres for anything else, with eaves capped at 2.5 metres in all cases.

The 2 metre distance is measured to the boundary of the curtilage, not to a fence line that sits inside it. If your fence is set 300 millimetres in from the legal boundary, measure from the boundary.

Verandas, balconies and raised platforms

A raised platform is defined as a platform more than 0.3 metres high, and providing one takes the development outside Class E.

This catches decking. A deck at 350 millimetres above ground, or a timber base raised to level a sloping garden, is a raised platform even where nobody would describe it as a balcony. It is also the reason that garden rooms on adjustable pedestal feet need their finished floor level checked rather than assumed.

The practical fix is to keep the finished platform at or below 300 millimetres, or to build up the ground beneath so that the platform itself is low. Note that regrading a garden can be an engineering operation in its own right if the change in levels is substantial.

Front gardens and designated land

Nothing under Class E may be built on land forward of a wall forming the principal elevation of the original house. That includes bin stores, bike stores and porches placed in a front garden, all of which need planning permission.

On designated land the side garden is also out of bounds, because nothing may sit between a side elevation and the curtilage boundary. On a detached house in a conservation area, that leaves only the rear garden.

The 20 metre rule on designated land catches large gardens. A studio at the bottom of a long garden in a conservation area is limited to 10 square metres of ground cover in total beyond that distance, which is a shed rather than a room. Materials and finishes are treated differently in these areas too, as with painting the exterior of a house.

Article 4 directions can remove Class E entirely. Check the directions register as well as the designation mapping.

Swimming pools, containers and other structures

A swimming pool is expressly within Class E, subject to the same limits, and its water area counts towards the ground cover. An enclosure over it, such as a pool house or a retractable cover, counts as well and will often be the thing that breaches the height limit.

Containers are included in the ground cover calculation, which is how a shipping container used for storage ends up being the difference between compliance and breach. A heating oil tank is permitted up to 3,500 litres capacity, above which it needs permission.

Ancillary equipment is treated under other classes. A heat pump, an electric vehicle charge point and solar panels each have their own class with their own conditions, including the projection limit covered in the 200 millimetre limit for domestic solar panels.

Setting the calculation out on a drawing

Put the arithmetic on the drawing rather than in a covering letter. A block plan at 1:200 or 1:500 showing:

1. The curtilage boundary, with its area stated.
2. The footprint of the original house, hatched and dimensioned, with its area stated.
3. Every existing structure, hatched differently, each with its area.
4. The proposed building, with its area.
5. A table giving the curtilage area less the house, the permitted half of that figure, the existing total and the proposed total.
6. A section through the building and the nearest boundary, with the 2 metre offset dimensioned and the height taken from the highest adjacent ground.

Then apply for a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990. It is the only formal way to confirm that works are lawful, it is decided on fact and law rather than on design, and it costs far less than resolving the question during a sale. A short Planning Statement explaining the calculation makes the officer's job straightforward.

Where the building fails one limit, a full planning application is usually granted for a well designed garden room in a rear garden. The difference is time and uncertainty, not impossibility, and a Design and Access Statement may be required by the local validation list.

This is general guidance, not legal advice. Permitted development rights are removed in some areas, so 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. Enter your postcode on PlanScope to see what your project needs.