You’ve decided to add an extension — maybe to open up the kitchen, or finally get that extra bit of living space. But before a single trench is dug, there’s a key question that shapes how the structure will stand: what type of house are you extending?
It might seem like a small detail, but whether your home is detached, semi-detached, or terraced can completely change how the new foundations are designed and built. Each type brings its own structural considerations — and its own risks if you get them wrong.
Detached homes: more freedom, more exposure
Detached homes are the simplest in many ways because they don’t rely on shared walls or neighbouring structures. That gives you more flexibility to design freely — you can choose your layout, foundation depth, and build sequence without worrying about party walls or shared supports.
However, that freedom comes with its own challenges. Because detached houses are fully exposed, they experience more lateral pressure from wind. This means foundations must be stable enough to resist twisting or settlement across the full footprint, without the lateral restraint that a neighbouring property provides. If you’re extending on multiple sides, consistency of foundation depth and ground condition becomes even more important to avoid differential settlement between old and new sections.
Semi-detached homes: balancing the shared wall
In a semi-detached house, one wall is shared with your neighbour — and that single connection affects everything from the structural design to the way the build is carried out.
The new foundations must take into account the load path through the shared party wall and ensure that any excavation nearby doesn’t undermine your neighbour’s structure. Often, this means following the Party Wall etc. Act 1996, which requires notice and sometimes agreements or monitoring during the works.
The other challenge is asymmetrical loading: the shared wall provides stiffness on one side, while the open side of your home is freer to move. Extension foundations need to account for this imbalance, making sure the new addition ties in properly without stressing the existing structure.
Terraced houses: sequencing and shared structure
Terraced homes are the most complex when it comes to extensions. Every wall, footing, and floor often connects directly to the next property — meaning your foundation work can easily affect your neighbours on both sides.
Working on a terraced property almost always involves party wall agreements and careful sequencing. You can’t just dig and pour concrete wherever you like; excavations must be planned in small, controlled sections to maintain the stability of the shared walls. In older terraces, you may even find that the existing foundations are shallow brickwork or random stone footings — not always suitable for supporting new loads without underpinning or replacement.
This type of house also means access is tighter, which affects machinery choice and foundation type. In some cases, engineers opt for piled foundations to minimise disturbance and prevent differential movement between houses.
End-terrace homes: a hybrid challenge
End-terrace houses sit somewhere between a terrace and a semi. Structurally, they still share a wall on one side, but they’re exposed on the other. That makes them vulnerable to the same lateral forces and movement risks as detached homes, while still having to respect the rules and sensitivities of a shared wall.
The foundation design for end-terrace homes often has to balance two conditions at once — protecting the stability of the neighbouring house on one side, while ensuring the new extension doesn’t move differently on the open side. If the existing walls are unevenly loaded or the ground conditions vary across the site, the engineer may need to reinforce or deepen one side of the foundation to keep the whole structure stable.
Final thoughts
Your house type doesn’t just affect layout or planning rules — it shapes the entire foundation strategy for your extension. Each brings its own structural challenges, from shared walls and boundary stability to differential movement and exposure. The key is to understand those differences early and design your foundations to suit. A good structural engineer will spot the risks before they become problems, keeping your build safe, compliant, and on solid ground.
