First Floor Extension Over a Garage

Planning rules, costs, and design considerations for building a first floor extension above your existing garage, including structural requirements and permitted development limits.

Building above an existing garage is one of the most efficient ways to add a bedroom, bathroom, or home office to your home. The footprint already exists - you are just adding a second storey on top. But the planning rules for first floor extensions are stricter than for single storey work.

Do You Need Planning Permission?

Permitted development

A first floor extension above a garage can sometimes be built under permitted development, but the rules are tight:

• The extension must not extend beyond the rear wall of the original house by more than 3 metres
• It must be at least 7 metres from the rear boundary
• Maximum eaves and ridge height must match the existing house
• The roof pitch must match the existing house as closely as possible
• No side-facing windows unless they are obscure-glazed and non-opening below 1.7 metres
• Materials must be similar in appearance to the existing house
• Not in a conservation area, AONB, or other designated land

Because many garages are at the side of the house, extending above them often involves building above the side elevation. Two storey side extensions under PD must:

• Not be wider than half the width of the original house
• Have a 1 metre setback from the side boundary at first floor level (i.e. the first floor cannot extend right to the boundary)

When you need planning permission

You will almost certainly need planning permission if:

• The garage is less than 7 metres from the rear boundary
• The first floor would be within 1 metre of the side boundary
• The extension would be wider than half the original house
• You live in a conservation area, AONB, or national park
• The design includes side-facing windows overlooking a neighbour

Structural Considerations

The biggest question is whether the existing garage can support an additional storey. In most cases, the original garage foundations and walls were not designed to carry a first floor. A structural engineer will need to assess:

• Foundations: are they deep and wide enough to support the additional load? You may need to underpin or add new foundations
• Walls: are they thick enough and in good enough condition? Single-skin blockwork (common in garages) cannot support a first floor. You may need to build new cavity walls alongside or replace the existing walls
• Roof structure: the existing garage roof will be removed entirely. A new first floor structure (steel beams and timber joists or concrete block-and-beam) will be installed

Budget for a structural engineer's assessment (typically £500-1,500) early in the project.

Typical Costs

Costs vary significantly depending on whether the existing garage needs demolishing and rebuilding, or whether the walls and foundations can be reused.

Common Uses

• Master bedroom with en-suite - the most popular option, adding significant value
• Home office - separate from the main house but still connected
• Teenager's room or guest bedroom - with its own bathroom for privacy
• Playroom or hobby room - using the garage below for storage

Design Tips

• Match the roofline - the first floor should look like a natural part of the house, not an afterthought. Match the ridge height, roof pitch, and materials
• Set back from the front - a slight setback (300-600mm) from the front of the garage makes the first floor look subordinate to the main house. Many planning officers prefer this
• Consider a side window ban - if you are close to the boundary, offering obscure-glazed or high-level windows on the side elevation can help get permission approved
• Retain the garage door - if you want to keep the garage as parking, you can build above it. The structural engineer will design the first floor to span the garage opening