Garage Extension Projects & Ideas – How Can You Extend Over a Garage?

“The age and type of property are usually a good indicator of the suitability of the foundations,” says Andrew Edwards, director of EDG Architecture. In the UK, the first set of national building standards was introduced in 1965. This did include a section on structural stability, so it’s possible garages built alongside houses from this era will have decent foundations so will be good contenders for a garage extension. “Garages built prior to 1965 might still have foundations, but they are likely to be shallower or smaller than required,” says Andrew.

For homes constructed after 1984, when Approved Documents were introduced to help clarify routes to meeting the Building Regulations standards, a garage’s foundations are even more likely to be suitable. “Post 1984, if a garage was constructed at the same time as the main house, it will usually have substantial foundations.”

Can Your Structural Walls Handle a Garage Extension? 

Full width cavity walls are the best-case scenario for building over an existing garage. These consist of an outer leaf of brick, stone or rendered block and an inner blockwork leaf, and most modern garages will have been built this way.

In this project by Paul Archer Design, the attached garage was extended over to accommodate a master bedroom and ensuite. The original second storey walls consisted of breeze blocks and timber cladding, so insulation was added to the outside and the timber replaced to form a cohesive external finish with closely matched roof tiles. Photo: Kilian O’Sullivan

“Older garages may have a single skin brick wall with brick piers,” says Andrew. “The brick piers provide stability to the wall and often this type of construction can still be built over, subject to the condition of the foundations.”

If your outbuilding does feature single skin brick walls, they should be assessed by a structural engineer, who will need to provide drawings to demonstrate they can support a garage extension. Any other type of structure may require more attention for a structurally sound garage conversion. “Timber-framed or concrete panel garages are unlikely to be suitable for building over without more significant works,” says Andrew.

What Are My Options if I Can’t Extend My Garage as it is?

There are a number of solutions if the existing foundations and walls are not suitable for building over, but which is the most appropriate for your garage is site specific.

In the event that the existing garage walls are unsuitable for extending over, the most obvious option is to rebuild them, giving you a clean slate to make them fit for purpose. However, it may be that you can retain the existing walls and build a new inner leaf of blockwork for a more cost-effective solution.

“In a modern cavity wall, the inner leaf carries the most load, while the outer leaf mainly provides weathertightness and an attractive finish,” says Andrew. “Providing it’s possible to create a load-bearing wall inside the garage, the importance of the existing wall is diminished.” Subject to the assessment of a structural engineer, the new inner skin can be built off the existing foundations, but if these are also unsuitable, it may be more cost effective to demolish and rebuild the garage.

Steel beams and columns can also be installed to provide a new frame, or if your garage has brick piers, these may be strong enough to avoid the need for new steelwork or adding an inner block wall. “The garage would remain single skin brick with a cavity wall on the extension above, offering significant cost savings,” says Andrew. “However, the brick piers may need underpinning. This is normally cheaper than building new walls if the existing ones are suitable, but it’s still expensive and complex.”

If the garage’s foundations are too shallow and require strengthening and the existing walls also need upgrading, underpinning may prove too costly and complicated. In this event, rebuilding the garage to modern performance standards is a more worthwhile investment if you’re considering a garage extension.

An alternative solution could be to reduce the weight of the extension over the garage. “Structural systems like masonry are heavy, so a timber frame or SIPs (structural insulated panels) extension could reduce the load and therefore the demand for structural alterations to the existing garage,” says Andrew.

Head Height & Insulation Considerations for Garage Extensions

Once you’ve assessed the structural suitability, factors such as head height and insulation will come into play. “The garage tends to be a cold space that isn’t designed to be weathertight,” says Richard Gill, director at Paul Archer Design. “So, you have to treat the floor of the storey above like an external wall and insulate it sufficiently, so you don’t lose all the heat in the room through it.”

Here is a garage conversion project getting underway, with alterations made to the original structure as a the old space is converted into a liveable zone

Head height and floor level must also be taken into account during the design process. “Garages are sometimes set at a different height to the second storey of the main dwelling,” says Richard. “You have to think about whether you’ll need to step up/down from the landing on the existing first floor into the new extension, or if you can create a level threshold.”

Depending on the ceiling height of the existing garage in relation to the first floor of the main property, you could reduce headroom in the garage below in order to make the floors level. “Adding insulation will also make the floor thicker. But in some cases, the head height in the garage may become too shallow to store a car, for example, so it may not be possible to get a level threshold,” says Richard.