Garage extension guide

What people mean by a garage extension

A garage extension can mean three quite different projects. The first is building a new garage attached to the side of the house. The second is extending an existing garage, usually to make it longer or wider so a modern car fits with room for storage or a workshop. The third is building an extension over a garage to add a bedroom or other room above. Each has its own structural and planning questions.

If you want to turn an existing garage into a room instead, that is a garage conversion, which our garage conversion guide covers separately.

Building a new attached garage

A new garage attached to the house is built much like a single storey extension: foundations, brick or block walls, a roof and a garage door, with a link into the house if wanted. Many older garages are too narrow for today's cars, so it is worth checking internal widths against the car you own and leaving space to open doors.

Matching bricks and roof to the house makes a garage look like part of the original building. A side garage often sits close to the boundary, so the Party Wall Act may apply to foundations near the neighbour's house.

Extending an existing garage

Lengthening a garage usually means adding to the back, keeping the front and the door. Widening it is harder, because the side walls, roof and sometimes the door opening all change. The existing foundations and brickwork need checking to see whether they can be tied into, and the roof often needs rebuilding as one so it does not leak at the join.

Building an extension over a garage

Adding a room over an existing single storey garage is a popular way to gain a bedroom without losing garden. The key question is whether the existing garage can carry the extra load. Many garages were built with shallow foundations and single-skin walls never intended to support a second storey. A structural engineer assesses the foundations and walls, often with trial holes. Some garages can be strengthened; others are better demolished and rebuilt as a proper two-storey extension.

Building Regulations require fire separation between a garage and the rooms above and beside it, with a fire-resistant ceiling and suitable doors. The new room also needs insulation, a safe escape window and a staircase or landing link from the main house, which affects the layout upstairs. Our double-storey extension guide covers the wider considerations.

Planning permission

Some single storey garages and side extensions can be built under permitted development, subject to limits on width, height and position relative to the road. An extension over a garage adds a storey, and two-storey side extensions are generally outside permitted development, so a planning application is usually needed. Councils typically look at how the extension relates to the neighbours and the street, and may ask for it to be set back or lower than the main roof. Our extension planning guide explains the approval routes.

Cost and timing factors

The state of the existing garage, foundation work, whether walls are rebuilt, the roof, fire separation, the garage door, and the fit-out of any room above all affect the price and programme. Access at the front of the house is usually good for garages, which helps. Our house extension cost guide covers more of the factors.

Our house extensions page explains how we build extensions from foundations to finish, and we offer free estimates after a site visit.

Questions people ask