Garage conversions: what the structure needs
Turning a garage into a room looks simple, but garages were rarely built to the standard of the house. Here is what an engineer checks, from the old door opening to the floor and foundations.
Do you need a structural engineer for a garage conversion?
Usually yes, for at least part of it. Converting a garage into a habitable room needs Building Regulations approval, and Building Control will want to know that the walls, foundations, floor and any new openings are adequate. An engineer typically designs the foundation and wall that replace the garage door, checks the lintel or beam above the opening, and confirms whether the existing foundations can take any new loads. If you are also opening the garage into the house, that wall needs a beam design too.
How much engineering is involved depends largely on the type of garage, its age and its condition. A sound integral garage in a 1960s house can be quite straightforward. A tired single-skin garage on clay near a tree is a different proposition.
Integral, attached and detached garages
An integral garage sits within the main footprint of the house, often with a bedroom above. The beam or lintel over the garage door then carries the front wall and part of the floor above, and its condition and capacity need checking whenever the opening changes.
An attached garage is built against the side of the house, usually single storey with a flat or lean-to roof. A detached garage stands on its own. Both were more likely to be built economically, with thin walls and modest foundations, because nobody expected them to become rooms.
Planning is a separate question. Many garage conversions fall within permitted development, but some estates had those rights removed by condition when they were built, and changes to the front of the house can matter in a conservation area. The council will confirm the position for your property.
Infilling the garage door opening
The old door opening is the centre of most conversions. It is replaced with a new wall, usually with a window, and that wall needs its own foundation. Garages often have nothing more than a slab edge or a shallow threshold across the doorway, so a new strip or trench fill foundation is normally dug between the existing walls or piers.
The new foundation should be designed for the ground, not simply matched to the shallow garage footings either side. On London Clay with trees nearby, that can mean going deeper than people expect. Where the new foundation meets the old ones, the detail needs to allow for small differences in movement, so a crack does not open up at the junction.
Above the opening, the existing lintel or beam was sized for a door that put no load on it. If it is corroded, undersized or a timber beam that has rotted, it may need replacing. This is particularly important in an integral garage with rooms above, where the beam is doing real work.
Walls, floors and roofs built to a garage standard
Garage walls are commonly a single leaf of brick or block, often half a brick thick with piers at intervals. To become a room they need insulation and protection from damp, usually by adding an insulated lining or building a new inner leaf. A new masonry inner leaf adds weight to foundations that may be shallow, so it is worth checking before choosing that detail.
Garage floors are normally a concrete slab, often lower than the house floor, sometimes sloping towards the door and frequently without a damp-proof membrane. The usual answer is a new membrane, insulation and screed over the existing slab, or a raised timber floor. Either way, the slab needs to be sound, with no sign of settlement or significant cracking.
Flat garage roofs were designed for light loads. Adding insulation, a plasterboard ceiling, a rooflight or solar panels can push the joists beyond what they were sized for, and an old felt roof may hide rot at the bearings. The engineer checks the joists and, where needed, specifies strengthening or a replacement roof.
Opening up into the house
Many conversions open the garage into the hall or kitchen to make a larger space. The wall between the garage and the house is often load-bearing, particularly with an integral garage, where it may carry the floor and walls of the rooms above. A steel beam on padstones is the usual solution, designed alongside the rest of the conversion. Our guide on removing a load-bearing wall explains what that involves.
That wall was also built as a fire separation between the garage and the house. Once the garage becomes a room the requirement changes, but Building Control will check fire safety and escape routes for the new layout, so it is worth discussing with your designer early.
Checking the existing structure first
Before the design starts, look at the condition of the garage. Common problems we find include:
Movement at the junction is common on clay, because garage foundations were often shallower than the house's, so the two respond differently to seasonal drying and wetting. If the movement is old and stable, it can be repaired as part of the work. If it is active, it should be understood before the garage is lined and decorated. A trial pit and foundation inspection report shows what the foundations are and what they sit on.
- Cracking where the garage meets the house, often caused by different foundation depths
- Leaning or bowing walls with little restraint from the roof
- Corroded steel lintels or rotten timber beams over the door
- Cracked or settled floor slabs
- Trees close to the garage on clay ground
What we provide
For most garage conversions, our extension structural package covers the infill foundation, lintels, openings, floor and roof, with calculations and drawings for Building Control, from £850 + VAT. If the only structural element is a single opening into the house, a load-bearing wall opening design from £400 + VAT may be all you need.
All our designs and calculations go through a three-step quality control process, with final sign-off by our Head of Engineering, and we send them to Building Control and liaise with them. Our typical turnaround is 10 working days, confirmed in writing with the fee. We see plenty of these in the interwar and post-war suburbs of boroughs such as Harrow.
Frequently asked questions
Does a garage conversion need Building Regulations approval?
Yes. Converting a garage into a habitable room is building work under the Building Regulations, covering structure, fire safety, insulation, ventilation and damp. Building Control will want structural details for the infill wall and its foundation, and for any beams or lintels. We send our calculations and drawings to Building Control and liaise with them on any structural questions.
Does a garage conversion need planning permission?
Often not, because conversion work that does not enlarge the house can fall within permitted development. However, some estates had permitted development rights removed by condition when they were built, and flats and listed buildings follow different rules. Changes to the front elevation can also matter in a conservation area. Check with your council before you commit to a design.
Can I build a room above my garage?
Possibly, but it is a different project from a conversion. Garage walls and foundations are rarely designed to carry a second storey. An engineer would expose the foundation in a trial pit, assess the walls and often design new foundations, an inner leaf or a steel frame to carry the load. A first floor addition also needs planning permission in most cases.
Why is there a crack between my garage and house?
Usually because the garage sits on shallower foundations than the house, or was added later without being tied in, so the two move independently. On clay soils, seasonal shrinkage makes this worse, especially near trees. A crack at the junction is common and often stable, but it should be checked before converting, because the new room will be lined and finished.
Will the garage floor need replacing?
Not always. If the slab is sound, reasonably level and not showing settlement, a new damp-proof membrane, insulation and screed can be laid over it, or a raised timber floor installed. Where the slab is cracked, sinking or too low to fit the build-up needed, breaking it out and casting a new insulated slab can be the better route.