A basement is the one part of a London house where the cost per square metre bears no relation to the rest of the building. Owners usually arrive with a rate taken from a finished scheme somewhere else, and that rate is almost always the wrong starting point. We are quantity surveyors in London and this is the single item we are asked to check most often.
The reason is simple. On a basement you are not paying mainly for the room. You are paying to hold the existing building up while you remove the ground underneath it, to get the spoil out through a terraced house, and to agree all of that with the people either side.
Underpinning is a sequence, not an item
The existing walls have to be supported before the ground goes. On a terrace that usually means underpinning in short bays, in a set order, with each bay cast and cured before the next one opens. The sequence is dictated by the engineer and it cannot be compressed by adding labour.
That sequence is the cost. A basement under a semi-detached house with clear side access and one under a mid-terrace with a single front door can carry the same floor area and very different prices, because one can be built in large bays with a machine and the other cannot.
Any rate per square metre that does not state the number of bays, the method and the access is a guess with a decimal point on it.
Temporary works are permanent cost
Propping, needling, shoring to the neighbour, protection to the party wall and the scaffold that supports all of it are temporary in the sense that they come out at the end. They are not temporary in the sense of being cheap.
On a constrained site the temporary works design is a separate engineering exercise with its own fee, its own drawings and its own programme. It is frequently missing from an early budget because nobody has drawn it yet, and it reappears at tender as a figure nobody expected.
Getting the spoil out
A basement generates a large volume of spoil and every cubic metre has to leave the site. Where a lorry can back onto a drive, that is a haulage line. Where the only route is a conveyor through the hall or a skip on a suspended licence at the kerb, it becomes a logistics operation with permits, banksmen and a working day that may be shorter than the contractor assumed.
Boroughs set their own rules on delivery hours, lorry routes, skip licences and pavement occupation, and they differ from one to the next. What applies to your street has to be checked against that borough rather than assumed from a scheme somewhere else in the city.
Waterproofing is a design decision with a price
Below ground you are designing against water, not just building a room. The usual approaches are a waterproof concrete box, an internal drained cavity system with a pump, an external barrier, or a combination of them. They do not cost the same and they do not carry the same maintenance obligation.
A drained cavity needs a sump and a pump, and a pump needs power, an alarm and somewhere to discharge. Those follow-on items are regularly left out of the first budget and then arrive as an extra once the structure is in.
The neighbours are part of the budget
Work of this kind normally engages the Party Wall etc. Act, which means notices, surveyors and an award before the relevant work begins. There may be more than one neighbour and more than one surveyor, and the timetable belongs to that process rather than to your programme.
The cost that matters is not only the fees. It is the condition surveys, the monitoring during the works, any protective works the award requires, and the delay if an award lands later than the programme assumed. We cover that in party wall awards and what they add to a build.
The ground decides more than the design
Two sites on the same street can sit on different ground. London clay behaves differently from made ground, and made ground with water moving through it behaves differently again. The method, the temporary works and the waterproofing all follow from what is actually under the house.
A ground investigation is modest relative to a basement and it is the one survey that changes the price rather than confirming it. Where it has not been done, every figure in the appraisal rests on an assumption that nobody has tested.
Groundwater is the item that most often turns a budget over. A high water table changes the excavation method, the pumping requirement during construction and the waterproofing specification afterwards, and it does all three at once.
What an honest basement budget contains
A budget we would put our name to sets out the structural method and the number of bays, the temporary works allowance, the spoil removal method and the borough constraints behind it, the waterproofing system and its follow-on items, the party wall process, and a contingency set against the things the survey has not yet answered.
It also states what it excludes. If the ground investigation has not happened, the budget says so on its face, because the difference between clay and made ground with water in it is not a detail. That approach is set out in cost planning and estimating.
What this means before you commit
If you are buying a house on the basis that a basement will pay for itself, the number in your appraisal should come from a measured scheme on that specific site, not from a rate. The gap between the two is routinely large enough to decide whether the project is worth doing at all.
Any figure quoted in conversation, including any figure you may see in an article, is an example rather than a price. Yours depends on the ground, the access, the neighbours and the borough.
Pricing a basement before you buy?
Send us the address, the drawings if there are any, and what you want the space to do. We will tell you what the basement is likely to carry and what has to be surveyed before anyone can be precise.