Outbuilding size and height limits under Class E

Outbuilding permitted development rules come down to six numbers and one condition. The numbers are 50 per cent, 4 m, 3 m, 2.5 m, 2 m and 0.3 m. The condition is that the building has to be used for a purpose incidental to the enjoyment of the house. Get all seven right and Class E of the General Permitted Development Order covers a shed, a garden office, a gym, a workshop, a summerhouse or a swimming pool without any application at all.

Get one wrong and the whole thing is unauthorised, because Class E is a package. There is no partial compliance.

The 50 per cent curtilage test, and how to calculate it

The rule reads: 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 total area of the curtilage, excluding the ground area of the original dwellinghouse.

Three things in that wording catch people out.

The original dwellinghouse means the house as it stood on 1 July 1948, or as first built if it was built later. Extensions added since then are not part of the original house, so their footprint counts towards your 50 per cent alongside the sheds.

The curtilage is the land around the house that serves it, which normally means the garden and drive but not adjoining paddocks or land in different ownership. It is a question of fact, and if you have bought a strip from a neighbour it may not be curtilage yet.

And existing structures count. The garage built in 1985, the greenhouse, the log store, the oil tank: all of them are in the total before your new garden room goes in.

Illustrative example. A plot of 400 m2 with an original house footprint of 90 m2 leaves a curtilage of 310 m2 for the calculation. Half of that is 155 m2. If a rear extension of 20 m2, a detached garage of 18 m2 and a shed of 6 m2 already exist, 44 m2 is used and 111 m2 remains. Almost nobody hits this limit on a suburban plot. On a small terraced garden or a flat's shared yard, people hit it regularly.

Which height limit applies to which roof

A dual-pitched roof has two slopes meeting at a ridge. A flat roof, a mono-pitch and, on the cautious reading most councils apply, a hipped roof all fall into the 3 m category, so the difference between a 4 m building and a 3 m one is decided by the roof form on the drawings.

Height is measured from the surface of the ground immediately next to the building, and where the ground is not level, from the highest part of that surface. On a garden that falls away from the house, the datum sits at the uphill corner, which quietly reduces the building you can put at the bottom of the slope. Survey the levels before ordering the structure.

The eaves limit of 2.5 m applies whichever height band you are in, and it is the one that catches flat-roofed garden rooms with a parapet. A parapet is part of the building.

The two-metre boundary trap

If any part of the building sits within 2 metres of any boundary of the curtilage, the whole building is limited to 2.5 m in overall height. Not the part near the boundary. All of it.

Two consequences follow. A 4 m gazebo positioned with one corner 1.9 m from the fence is not permitted development, even if the rest of it is 5 m away. And moving the building 100 mm can be the difference between a lawful 3 m structure and an unlawful one, which is why the position should be dimensioned on the site plan from a fixed feature rather than eyeballed on site.

The boundary in question is the boundary of the curtilage of the dwellinghouse, which is not always the fence. Where a strip of your land sits outside the curtilage, or where the legal boundary runs along the centre of a hedge, measure to the right line.

Forward of the principal elevation

Nothing under Class E may be sited on land forward of a wall forming the principal elevation of the original dwellinghouse. The principal elevation is normally the front of the house, the one containing the main architectural treatment and usually the main entrance, and it is fixed by reference to the original building rather than to any later extension.

The restriction is about the plane of that wall extended sideways, not just the area directly in front of the house. On a corner plot the side garden can be wholly forward of the principal elevation, which is why bin stores and log stores on corner plots are a recurring enforcement subject.

Where a house faces away from the road, or where a later extension has changed which elevation reads as the front, the answer is not obvious. That is a good case for a certificate rather than an assumption.

Verandas, balconies and raised platforms

Class E does not permit a veranda, a balcony or a raised platform. A raised platform is one more than 0.3 m above the ground, so a decked terrace at 250 mm is fine and the same deck at 350 mm is not.

This is the limb that catches garden rooms on sloping plots, where the structure is levelled on a raised deck to avoid excavation. The building may be within height limits and the deck still takes the whole thing outside Class E. It also catches the steps and platform arrangements often supplied with hot tubs and outdoor kitchens.

Containers have their own limit, capped at 3,500 litres capacity, which is the rule that governs domestic oil tanks.

Conservation areas, National Landscapes and listed buildings

On land in a conservation area, a National Park, a National Landscape, the Broads or a World Heritage Site, two further restrictions apply. Nothing may be sited between a side elevation of the house and the boundary of the curtilage. And the total ground area of buildings, enclosures, pools and containers situated more than 20 metres from any wall of the house must not exceed 10 m2.

That 20-metre rule is the one people have not heard of. On a long garden in a conservation area, a 12 m2 garden office at the far end is outside Class E even though it meets every height and coverage test.

Within the curtilage of a listed building, Class E does not apply at all. A shed next to a listed house needs planning permission, and works affecting the special interest of the building or its curtilage structures may need listed building consent as well, which is a criminal offence to carry out without. If your house is listed, or you are unsure whether an outbuilding is a curtilage structure, take advice before you order anything and expect to need a heritage statement with the application.

Article 4 directions remove permitted development rights across defined areas, and planning conditions on newer estates often remove them plot by plot. Check the title deeds and the original estate permission as well as the council's map.

Confirming it rather than hoping

The condition attached to Class E is that the building is used only for a purpose incidental to the enjoyment of the house. Sleeping accommodation, an independent kitchen and a self-contained layout take a building outside that condition however small it is, and that is a separate question from the dimensions covered here.

Where the numbers are tight, or the principal elevation is ambiguous, or the curtilage is unclear, apply for a Lawful Development Certificate for proposed development. It is the only formal way to confirm that works are lawful, it produces a decision you can show a buyer's solicitor, and it costs far less than removing a completed building. Where the answer is that permission is needed, a householder application with a short planning statement covering siting, scale and materials is usually straightforward. If you are also extending the house, the rear extension depth limits interact with the coverage calculation above, so work both out together.

This is general guidance, not legal advice. Permitted development rights vary by site and can be removed by condition or Article 4 direction. Check the position with your local planning authority or a qualified professional before relying on it.

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