Building a garage without planning permission is possible under the permitted development rights for outbuildings that apply to dwellinghouses in England. The size of the garage you can build without planning permission is defined by height limits and a coverage rule rather than by a specific maximum floor area.

The Permitted Development Rules for Garage Size

Under Class E of Part 1 of the General Permitted Development Order, a detached garage is an outbuilding that can be built without planning permission provided it meets the following conditions.

The garage must not exceed 2.5 metres in eaves height. The eaves height is measured from the ground to the point where the roof slope meets the wall, not to the ridge. The total height must not exceed four metres with a dual-pitched roof, or three metres with any other roof type including flat and mono-pitch roofs. The garage must not be positioned forward of the principal elevation of the house or to the side of the house where that side faces a highway. The combined footprint of all outbuildings and extensions on the property must not cover more than 50 percent of the total area of the original curtilage of the house, excluding the footprint of the original house itself.

What This Means in Practice

A standard single garage with a pitched roof would typically have eaves at around 2.2 to 2.4 metres and a ridge at around 3.5 metres, comfortably within the limits. A double garage with a pitched roof at similar proportions would also be within the permitted development height limits. The primary constraint on garage size under permitted development is therefore the 50 percent curtilage rule rather than the height limits for most standard residential plots.

On a plot with a small garden, the 50 percent rule can become a limiting factor relatively quickly, particularly if there are already other outbuildings or extensions. On a larger plot, a substantial garage can be built under permitted development without approaching the coverage limit.

Internal Height and Floor Area

There is no specific maximum floor area in the permitted development rules, only the coverage limit. A garage that occupies, say, 60 square metres of ground floor area is permitted development on a plot large enough for it to remain within the 50 percent rule, provided it meets the height conditions. A garage with an internal height of around 2.4 metres to the eaves is typical for a single-storey structure within the limits. Garages taller than 2.5 metres to the eaves require planning permission.

Building Regulations

A detached garage under 30 square metres with no sleeping accommodation is generally exempt from Building Regulations. A garage between 15 and 30 square metres that is within one metre of a boundary and is not of substantially non-combustible construction requires Building Regulations approval for fire-resistance of the boundary wall. Garages with electrical installations must comply with Part P. Garages used for any purpose beyond storage and vehicle parking may require Building Regulations approval for the specific use.

Summary

Under permitted development, a detached garage can have eaves up to 2.5 metres and a total height of up to four metres with a pitched roof. There is no maximum floor area rule, but the garage and all other outbuildings and extensions combined must not exceed 50 percent of the original curtilage area. The garage must be in the rear or side garden, not forward of the house's front wall. Building Regulations apply to garages over 30 square metres and to all electrical installations.