What Is an Outbuilding?
For planning purposes, an outbuilding is a separate structure within the curtilage (land around) a dwelling house, that is incidental to the enjoyment of the dwelling. This definition includes garden offices, home studios, garden rooms, garages, workshops, garden storage buildings, swimming pool enclosures, and similar structures.
The key word is incidental — an outbuilding is ancillary to the main house, not a separate dwelling. An outbuilding that contains sleeping accommodation or is used as a self-contained unit of accommodation does not qualify as incidental use and requires a separate planning application regardless of size.
Permitted Development Rights for Outbuildings
Under Class E of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, outbuildings within the curtilage of a dwelling house can be built without planning permission provided they meet specified conditions.
The main conditions are: the outbuilding must not be in front of the principal elevation of the house (the elevation facing the road). The eaves height must not exceed two and a half metres. If the outbuilding has a dual-pitched or mono-pitched roof, the maximum ridge height is four metres for a dual-pitched roof or three metres for any other roof form. If the outbuilding is within two metres of the boundary of the curtilage, the maximum overall height is two and a half metres. The outbuilding must not cover more than fifty percent of the curtilage of the original house when combined with other extensions, outbuildings, and structures. The outbuilding must not contain a microwave antenna or a chimney (though small flues are sometimes permissible).
For properties in designated areas (national parks, areas of outstanding natural beauty, World Heritage Sites) and for listed buildings, stricter rules apply and outbuildings over ten cubic metres may require planning permission.
Garden Offices and Home Studios
The growth in remote working has created significant demand for garden offices — high-quality insulated, heated structures designed for full-time professional use. These can typically be built under permitted development provided they meet the conditions above. For planning purposes, a garden office used incidentally to the main dwelling (the occupant works there, it is not rented out or used commercially) generally qualifies as an incidental outbuilding.
For buildings that are intended to be used commercially, rented to others, or used as business premises separate from the main household, the incidental use argument becomes more difficult to sustain and a change of use application may be required.
Building Regulations and Outbuildings
Permitted development status does not remove the need for building regulations in all cases. Small, single-storey outbuildings with a floor area under fifteen square metres are generally exempt from building regulations. Between fifteen and thirty square metres, they are exempt provided they are not used for sleeping accommodation. Over thirty square metres, building regulations approval is typically required. For insulated, heated garden offices or studios, it is advisable to confirm the position with building control even in the exempt size ranges, particularly where mains electrical connection is involved (which often triggers Part P).
Outbuildings and Planning Applications
Where an outbuilding does not qualify for permitted development — because of size, position, or the constraints applying to the specific site — a planning application is required. Outbuilding planning applications are generally straightforward provided the proposed structure is reasonable in scale relative to the garden and house, and does not create unacceptable impacts on neighbours.
At RCB Design and Build, we regularly build garden offices, studios, and ancillary outbuildings as part of wider garden and extension projects. We advise on permitted development compliance, building regulations position, and planning requirements before work starts. Contact us to discuss your outbuilding project.