The short answer
A garage conversion is usually the cheaper and quicker way to gain a room, because the walls, floor and roof already exist. A single storey extension costs more and takes longer, but it adds new floor area to the house instead of repurposing space you already have. If you have a sound, attached garage and need one extra room, conversion is often the sensible choice. If you need a bigger or better-shaped space, or you want to keep your garage, an extension is the better route.
How they differ
A garage conversion turns an existing garage into a habitable room, such as a bedroom, home office, playroom or utility space. The main work is usually replacing the garage door with a proper wall and window, insulating the floor, walls and roof, and fitting heating, lighting and sockets. The footprint of the building mostly stays the same.
A single storey extension is new construction on the ground floor, usually at the back or side of the house. It needs foundations, new walls, a roof and a connection to the existing house. You get to decide the size and shape within the limits of your plot, and it can be designed to suit the way you live, for example a kitchen-diner with large glazed doors.
A few practical differences follow from this:
- Size: a conversion is limited to the size of the garage. An extension is limited only by your plot, your budget and what the planning rules allow.
- Layout: garages are often attached to the side of the house, so the new room may not sit where you would ideally want it. An extension can be placed where it works best.
- Disruption: a conversion is mostly contained within an existing shell. An extension involves groundwork, an opening in the house wall and more time with the property open to the weather.
- Loss of the garage: a conversion removes your garage. An extension keeps it.
Both usually need building regulations approval. Planning permission is sometimes needed as well, depending on the property, its location and any conditions attached to the original planning consent. Many garage conversions do not need it, but some do, so check with your local council before you commit. If an extension is close to a neighbour's boundary, the Party Wall etc. Act may also apply.
Cost: what makes each more or less expensive
In general, a garage conversion costs less than a single storey extension of a similar size, because you are not paying for foundations, a new roof or most of the structure. That said, the final price of either project depends on the details.
What pushes a garage conversion up:
- A damp, uneven or poorly built floor that needs significant work.
- Thin single-skin walls that need substantial insulation and finishing.
- Adding a bathroom or kitchen, which means plumbing and drainage.
- Replacing the garage door area with a wall that matches the house well.
- Rerouting electrics or moving a boiler or meter that sits in the garage.
What pushes an extension up:
- Size, since every extra square metre adds materials and labour.
- Ground conditions and the depth of foundations needed, including nearby trees or drains.
- Large glazed doors, rooflights and other high-end features.
- A kitchen or bathroom fitted in the new space.
- Knocking through a large opening in the existing house and supporting it with steelwork.
- Matching brickwork and roof finishes to the existing house.
The relative cost per usable room matters too. If you only need one extra room, a conversion is hard to beat on price. If you want a large open-plan space, an extension may give better value for what you get, even though the total is higher. Check the real prices other homeowners paid for each type of job shown on this page, as they give you a more reliable guide than general rules of thumb.
Think about the effect on your home's value as well. Losing a garage can matter more in areas where parking is scarce, while an extension that adds a well-planned living space is usually easier for buyers to understand. Local estate agents can tell you how buyers in your area view each.
Lifespan and upkeep
A well-built conversion or extension should last as long as the rest of the house, provided it is built properly and maintained. The difference is mainly in what you are starting with.
A conversion depends on the condition of the original garage. Garages were often built to a lower standard than the house, with shallower foundations, thinner walls and basic roofs. If the structure is sound, that is fine. If there are cracks, damp or a worn flat roof, those problems become yours to deal with. The infilled area where the door used to be is also worth checking over time for cracking or water getting in.
An extension is new, built to current standards, and is usually better insulated as a result. The main upkeep points are the same as for any part of a house: the roof, especially if it is flat, rooflights, window seals, gutters and external joints. Flat roofs and large glazed areas are the parts most likely to need attention over the years.
In both cases, quality of workmanship matters more than the type of project. Poor insulation, badly sealed joints or weak damp-proofing will cause trouble in either.
When to choose Garage conversion
A conversion is likely to be the better choice when:
- You need one extra room rather than a large new space.
- Your garage is attached, structurally sound and dry.
- You rarely use the garage for parking, or you have a drive or street parking that suits you.
- You want a shorter, less disruptive project.
- Your budget is tight and you want the lowest-cost way to add usable space.
- The room will sit comfortably next to the rest of the house, for instance a study or guest bedroom beside the hall or kitchen.
When to choose Single storey extension
An extension is likely to be the better choice when:
- You want a larger or more flexible space, such as a bigger kitchen or family room.
- You want to keep the garage for parking or storage.
- Your garage is detached, small, in poor condition or awkwardly placed.
- You want the new space to connect well with the garden and the rest of the ground floor.
- You are planning for the long term and are willing to spend more for a layout that suits you.
- Losing the garage could make the house harder to sell in your area.
Questions to ask a tradesperson
- Do I need planning permission, or only building regulations approval, for this project?
- Who will deal with building control, and will I get a completion certificate at the end?
- What condition are the garage's foundations, floor and walls in, and what work will they need?
- How will the floor, walls and roof be insulated, and how will damp be kept out?
- What is and is not included in the quote, such as electrics, plastering, flooring and decorating?
- What might change the price once work starts, and how will you tell me about it?
- How long will the work take, and which parts will cause the most disruption?
- Can you show me similar jobs you have completed and put me in touch with past customers?
- What guarantees or warranties do you provide on the work?