The quote you accept and the bill you pay are often different numbers, and the gap usually opens underground. Most of it comes down to what the ground and the existing house reveal once work starts, which is why any architects bromley homeowners appoint will push for a proper survey before anyone prices the job.
These are the problems that surface most often on older houses, what each one costs, and how to catch it before it turns into a variation.
Shallow Existing Foundations
This is the most common one, and older housing stock is where it lives.
Modern foundations run around a metre deep. Victorian, Edwardian and interwar houses were frequently built on foundations 400 to 600 millimetres deep, sometimes less. That was fine for the original house. It becomes a problem when you build a new, deeper extension foundation right next to a shallow old one, because the two behave differently under load.
The fix is often underpinning the existing foundation, or designing the new one to avoid loading the old. Underpinning is not cheap. Mass concrete, the standard method for shallow foundations, runs roughly £1,500 per square metre, with London prices typically around 20 per cent higher. On a whole extension flank that can add £5,000 to £15,000, and a full underpinning project runs far higher.
How to catch it early. A trial pit at survey stage. Digging down beside the existing wall to see the actual foundation depth costs very little and tells you whether underpinning is even in the conversation. Skipping it is how the number moves after work starts.
Clay Ground and Tree Roots
Bromley illustrates this one well. Much of the borough’s Edwardian and 1930s stock sits on generous, leafy plots, and that combination of clay soil and mature trees is exactly what complicates foundations.
Clay shrinks and swells with moisture. A mature tree within influencing distance draws water out of the clay, which moves the ground, which is why foundations near trees have to go deeper. Standard strip foundations at a metre can rise to 1.5 metres or more, and where the ground is genuinely difficult, the design shifts to trench fill, raft or piled foundations, each adding cost.
Foundations and base works can account for 25 to 40 per cent of a single storey extension budget once ground conditions are difficult, against a much smaller share on straightforward ground. That is a large swing to discover late.
How to catch it early. A soil investigation and an accurate tree survey before the foundation is designed. The engineer needs to know the soil type and the position of every significant tree, because both change the required depth. On a leafy Bromley plot this is not optional detail, it is the main cost variable.
Half Brick Party Walls
A quieter problem, and specific to older terraces and semis.
Some period houses were built with party walls only half a brick thick, where a modern design would assume a full brick wall. When you cut into that wall to support new floor joists or a beam, a half brick wall carries load differently and may need additional support that a full brick wall would not.
It rarely stops a project, but it changes the structural detail and the cost, and it is invisible until someone opens the wall up.
How to catch it early. The measured survey should establish party wall construction, not assume it. On a semi where you are going into the shared wall for a loft or a side extension, this is worth confirming before the structural design is finalised.
Undersized Existing Roof Structure
This one shows up on loft conversions specifically.
The existing ceiling joists in most older roofs were designed to hold up a ceiling, not to carry a floor with people and furniture on it. They are almost always undersized for habitable use, which is expected and designed for. The surprise is when the rafters themselves are undersized or the roof has been altered at some point in a way the drawings do not show.
New floor beams and a ridge beam are standard on a loft conversion and get priced in. The unbudgeted version is discovering the existing structure is in worse condition than assumed, or that a previous owner removed something they should not have.
How to catch it early. A proper loft survey before design, checking rafter sizes and the condition of the existing structure rather than working from the original drawings.
Chimney Breasts Removed Below, Stack Left Above
A problem that arrives from previous owners rather than the ground.
If someone removed a chimney breast on a lower floor at some point without properly supporting the stack above, you can have masonry sitting on inadequate support. It might have stood for years, but an extension that changes loads nearby can bring it into focus, and Building Control will want it addressed.
Supporting a stack with a gallows bracket arrangement is a designed solution, not an assumed one, and it needs calculating.
How to catch it early. Check the planning and building history, and have the survey look specifically at whether breasts have been removed and how the stack above is supported.
What Ties These Together
Every one of these is cheaper to find at survey stage than after work starts, and the reason is leverage.
Found during the survey, a problem is a line in the design and a number in the quote. Found mid build, it is a variation, priced by the builder when the scaffolding is up and you have nowhere else to go. The same fix costs more the later it appears, and the emotional cost of a stalled project on top of that is real.
This is where having the structural engineer involved from the start earns its place. Most practices outsource structural work, which means the survey, the design and the calculations pass between separate parties, and gaps open in the handoffs. Extension Architecture keeps a structural engineer in the same team as the design staff, so the survey findings feed straight into the design while it is still fluid. A structural engineer surrey projects rely on will want soil conditions, tree positions and existing foundation depths established before designing anything, because on the clay soils common across Surrey and outer London, those three factors decide the foundation cost.
The One Thing Worth Spending On Before You Build
A contingency. Holding 10 to 15 per cent of the build cost back for the unforeseen is standard practice, and on a period property it is not optional.
A quote with no contingency is not cheaper. It is less honest about what an older house can hide. The projects that come in on budget are the ones where somebody investigated the ground and the existing structure properly before the price was fixed, and held a sensible reserve for whatever the survey could not reach.
Older houses keep their secrets underground and inside their walls. The money is well spent finding them before the builder does.
