Converting a garage: what you need to know - Grand Designs Magazine

Converting a garage: what you need to know

Choosing to convert your garage could be a great way of extending your current living space

By Becca Green |

A garage conversion is a popular option to extend your current living space. If you’re considering taking on the project yourself, here are a few things to consider.

A garage can be a huge selling point for a home, but if you also have off-road parking, you may find that this space has become the designated dumping ground for the family.

If you need more space and want to improve, not move, a garage conversion could be an effective and affordable option that can be enacted quickly and add significant value to your home.

Here’s what you need to know before starting out on your garage renovation journey.

Archmngers Stego garage conversion - grand designs

Archmongers Stego garage conversion. Photo: French + Tye

Planning permission

Check with your local authority to see whether a planning application is required. In many urban areas, permitted development rights have been revoked for garage conversions as councils are worried about the knock-on effects of people parking on the street. Even if it is considered permitted development, always obtain a Lawful Development Certificate.

If permitted development rights have been removed, you can still make a full application, which would also be necessary where larger external changes are planned.

As well as knocking through into an attached garage, it’s possible to add a storey on top – although this isn’t as simple as it seems. Planning permission would be required, and planners don’t favour such projects in streets of semi-detached houses because of the potential for the whole road to have a monotonous flat facade – the so-called ‘terracing’ effect.

Strong foundations

The majority of these structures lack strong foundations, in which case you’d need to knock it down and rebuild. Even if the foundations are sound, you’ll need to factor in upgrading the fabric of the garage, such as by adding lots of roof insulation.

Garage conversion project by OB Architecture. Photography: Martin Gardner

Garage conversion project by OB Architecture. Photography: Martin Gardner


Converting a garage into a liveable space requires warmth, therefore it must have insulation. One of the most crucial factors when beginning to insulate a garage is making sure it’s completely airtight. Garages aren’t built to contain heat and more than likely you will find lots of airgaps leading to the outside. You can insulate the walls, ceiling, and doors of the garage but if you fail to fill the gaps, it will be a big struggle to keep the area heated.

The three main forms of insulation for a garage are:

  1. Fibreglass insulation
  2. Cellulose insulation
  3. Foam insulation

Ultimately the decision is up to you when it comes to deciding which insulation material to use. There are many different types of insulation to which satisfy different objectives.

de rosee sa garden room converted outbuilding - grand designs

This new garden room, designed by De Rosee Sa replaces an outbuilding and includes a small kitchen, studio bed, shower room and toilet. Photo: Alex James

Making use of the existing structure

Replacing the garage door with a window is relatively easy because you can use the existing lintel, then infill using a similar material, whether that’s brick, render or stone. If converting a single garage, make the long, narrow proportions of the room work to your advantage by splitting the space in two. The rear area could be used as a utility room accessed off the kitchen or as an en suite.

Creating a link

When converting an attached garage, it should be relatively easy to link more cohesively with the rest of your property, whether you knock through or choose to retain a door to the space.

If the garage is relatively close to the house, the decision to link them depends on the way you live and how you intend to use the space in the future. Adaptability is also important – it might start off as a playroom for your children, but have the potential to become a media room or home office in years to come.

A separate garage conversion may provide a better option for an annexe. You will also need to factor in providing utilities to the garage, including electricity, if it doesn’t already have it, as well as water if you’re looking to create a self-contained space.

Archmongers Stego garage conversion: photo FRENCH TYE

Architect Archmongers extended a 1980s end-of-terrace in south London, linking the property to a bedroom that was converted from a garage at the end of the garden. This radical project cost £210,000. Photo: French + Tye


According to MyJobQuote, the average cost of a garage conversion is £13,500, but this could vary as much as £7,500 to £20,000 depending on the complexity of the project. The largest costs involved are ensuring the foundations are strong enough and infilling the garage door.

The average project lasts 2-4 weeks.