1 and 2. Under or tight against something already there.
The problems in this Evening Standard article, costing an average of £24,000 to sort out, are all concerned with basements burrowed under houses already there or tight against the back of a house already there.
The difference in safety issues between:
Add to that access room: spacious or tight,
working room: spacious or tight,
dry ground, wet ground or running sand
and the variety of cost for the same basement in terrible conditions could be 100 times that in easy conditions.
Murphy's law applies:
Where land values are the highest, the water table is the highest as well and the soil the very worst: running sand. Also, these places have the least room around them as well.
So in Kensington and Battersea where it would be extremely dangerous they find it economic to build a basement for £1 million that would be uneconomic for £60,000 in Norfolk - where it would be much safer.
Please don't under-estimate the cost in money, human life and your neighbour losing their home if you dig deep under a house into tidal Thames water and running sand without being able to pile first. Unfortunately, being able to pile the entire perimeter first when you have a terraced house is likely to be impossible. I have not done it. I would not do it. I will not help you go under anything already there close to the Thames.
3. New, Empty or lived in?
An entirely new dwelling can be zero rated for VAT, though you still pay VAT on anything not a part of the build, such as tool hire, design and management.
An empty dwelling might be refurbished for only 5% VAT.
A dwelling already lived in and being extended suffers the full 20% VAT.
5. How does shape affect price?
You can see that the basement just above has the minimum 4 sides and the minimum 4 corners.
This means very little timber was cut to get the dimensions right so lots of it was re-used, saving money later.
6. Proximity of highways.
If you excavate anywhere near a public highway you introduce another level of bureaucracy. Unfortunately highway authorities have their budgets cut so they have no expertise left in house, which makes them more difficult to deal with than they used to be. They will make you pay for private consultants to check all your calculations and so on.
Trees dry ground out during spring and keep it dry through the summer.
So you need to know your soil to plan the best time to dig it and have it carted away.
8. Using products with BBA certificates unnecessarily.
Part 3 is the test for compressive strength.
Part 8 is the test for permeability of concrete by water under pressure.
Yet your architect and your engineer are both likely to specify a concrete admixture they think waterproofs concrete but doesn't, because it has a BBA certificate.
And because it has a BBA certificate the producers think they can charge a fortune for something proven not to work.
The absolute truth is that the BBA certified admixtures are added to an already impermeable mix of concrete and do little or nothing. Many are mostly just a bit more cement for a lot of money. Most of the others provide no benefit.
The producers send people to present to architects and engineers who seem, unfortunately, easily hoodwinked.
I will help you buy waterproof concrete for less money and get your evidence, a test certificate to BS EN 12390 part 8, that the concrete you used in your project is impermeable.
9. Can I have a basement in the water table, for instance, down at the same level as the Thames?
See a 19 second video showing what happens down at water level if you dig or auger , such as CFA piling, into or through the water bearing layer here.
The short answer is Yes You Can If Your Pockets Are Deep Enough.
It will swallow money because arrangements have to be made, such as a perimeter diaphragm wall, steel sheet or secant piling or pile casings, to stop the ground collapsing in sideways as you extract gravel from water.
If you allow any material at all to come in from the sides then the ground around is going to have to settle and you might cause subsidence to neighbouring properties.
You have to find a way to control and reduce the water coming in to your excavation if you are close to and lower than a nearby river.
The basement really should be built so it is waterproof so that other waterproofing measures are not overwhelmed.
Care must be taken with the maths to be sure that the basement could not float and rise, not least because it would settle unevenly.
You would need to make sure there wasn't a way in for water if the nearby river flooded.