Foundations for Homes, Garages and Extensions: What You Need to Know
When you build a new home, add a garage or extend your house, one of the biggest decisions is the foundation underneath it. At L Groundworks, we know foundations are not one size fits all. What works for a house will not always work for a garage or an extension, and getting this right matters for how long your build lasts.
What foundations actually do
Foundations are the link between your building and the ground. They spread the weight of the building evenly across the soil, so the structure does not sink, move or crack over time. The right foundation for your project depends on the soil, the weight of the building, the local weather and what the structure is for.
In the UK, building regulations and British Standards set out what foundations need to do. These rules cover ground conditions, frost protection and load, and they vary by project type. Extension footings, for example, often need to meet different standards to the footings of the original house, which is why foundation work on an extension needs careful planning. You can read more about when building regulations approval is needed on gov.uk.
Foundations for a home
A home needs foundations built to last for decades. They must support several storeys, the everyday weight of people and furniture, and stand up to ground movement and weather over many years.
Most UK homes use strip foundations, long strips of concrete running under every load-bearing wall to spread the weight of the house evenly into the ground. Strip foundations usually go down at least 450mm, which keeps them below the frost line and stops the ground freezing underneath from pushing the foundation upwards.
Where the soil is difficult, such as clay that shrinks and swells with the weather, deeper foundations or trench fill foundations are often needed instead. If you want to weigh up the options for your own site, our guide to raft, strip and pile foundations breaks down which type suits which project.
Homes also need damp proofing, insulation and space for drainage and other services running underneath the floor, all of which have to meet current building regulations.
Garage footings and foundations
Garage foundations are usually simpler than home foundations, mainly because a garage is a single-storey building with lighter, more predictable loads. A garage mostly needs to hold up a vehicle, some storage and a basic roof structure, rather than the multi-storey loads a house has to carry.
For a detached garage, a concrete slab foundation is often the best option: a reinforced concrete pad that acts as both the foundation and the garage floor, usually at least 100mm thick and reinforced with steel mesh so it can cope with the weight of a car and anything stored on top.
Garage footings and foundations still need to go below the frost line, even though they are shallower than a house foundation, and good drainage is essential since garages often deal with water off cars and rain from outside. An attached garage adds another layer, since its foundation has to work alongside the existing house foundation while carrying its own separate loads.
Extension foundations
Extension foundations are often the trickiest of the three, because they have to tie in neatly with a foundation that is already there. The main challenge with footings for extension work is matching the depth and type of the new foundation to the old one, so the two parts of the building move together rather than against each other.
If the original house has shallow foundations, building an extension with much deeper foundations can create weak points where the two structures meet. The same problem can happen the other way round, so extension footings usually need to be designed around what is already there. Our guide on how to choose the right foundation for your extension covers this in more detail.
Current building regulations often set a higher standard than the one the original house was built to, which sometimes means the foundations for a single-storey extension end up stronger than the foundations of the house itself.
Extensions, especially side return or rear extensions, sometimes need underpinning of the existing foundations too, strengthening them so they can safely carry the new extra weight.
Soil and site conditions
The soil underneath makes a big difference to any of these projects. Clay soil, common across much of the UK, shrinks and swells as moisture levels change. Sandy soil usually drains well and gives stable support, though wider foundations are sometimes needed to spread the load. Rocky ground can bear a lot of weight but is harder to dig into and may need extra equipment.
A proper soil test before work starts gives the information needed to choose the right foundation, including how deep to go and how the site will drain. We explain why this step matters so much in our piece on why soil testing is crucial for building foundations.
Getting it right
Choosing the right foundation means weighing up the soil, the building regulations and how the structure will be used for years to come. At L Groundworks, we assess all of this for every project, whether it is a new home, a garage or an extension, so the foundation underneath is built to last. If you are lining up quotes, our foundation contractor checklist is a useful place to start before you commit.
Working with an experienced groundworks team means your foundation will support your build safely for decades, while meeting every building regulation along the way. Good foundations are worth getting right, since they hold up everything that comes after them.
FAQ:
What is underpinning?
Underpinning means strengthening or extending an existing foundation, usually by adding new concrete beneath or beside it. It is often needed on extensions where the original foundation is not deep or strong enough to carry the extra weight safely.
What is a raft foundation?
A raft foundation is a single slab of reinforced concrete that covers the whole footprint of a building, spreading the load evenly across the ground rather than through separate strips or piles. It suits softer or less consistent soil where strip foundations are not enough on their own.
How much does underpinning cost?
Underpinning costs vary a lot depending on the depth needed, the length of wall involved and the ground conditions on site, so it is best confirmed with a site survey and a written quote rather than a fixed figure.
How deep should foundations be?
Most UK strip foundations for a house go down at least 450mm, though the exact depth depends on the frost line, the soil type and what the building regulations require in your area. Garages and single storey extensions are often shallower, while difficult soil such as clay can call for deeper foundations.
How deep should foundations be for a single storey extension?
Foundations for a single storey extension usually need to match the depth of the existing house foundation rather than follow one fixed measurement, so the two structures move together as one. A groundworks contractor checks the existing foundation depth and the soil before confirming the depth for the new work.
Do I need building regulations for a garage?
A detached garage often falls outside full building regulations if it stays under a certain size and is not used as living space, but an attached garage or one connected to services usually does need approval. It is worth checking with your local authority or a groundworks contractor before you start.