Adding a basement?

With news that a property owner in Hampstead is planning to more than double the size of his house by constructing a vast basement, we are reminded of a survey we did several years ago. Our client owned a very nice newly refurbished house in Hampstead and intended doing something similar. You wonder why these owners want the aggro and difficulty, not to mention the expense of such projects.

Image inside basement undergoing renovation

The answer generally is that it is a cheaper option than trying to buy a bigger house and convert it to your needs. In fact, basement construction is all the rage in London right now – for houses large and small. And with it, comes a bundle of potential difficulties and traps for the unprepared.

The process of constructing a basement is actually quite straightforward – underpin the existing walls (including any wall shared with a neighbour), dig out the hole, cast a floor slab and fit out the space.

But the building regulations now have stringent thermal performance rules that have to be met as well as fire and structural requirements – this type of work and design is not easy and should be reassuringly expensive!

So what could go wrong?

Well, digging a great big hole next to a house isn’t a great idea because occasionally things like living rooms fall into it! The underpinning has to be spot on and properly designed and the amount of earth that needs to be dug out should not be underestimated – the rule of thumb is that it bulks up to three times its volume in the ground so there’s a tendency to overfill skips. The skips get too heavy and then… kaboom… they collapse into the road or vaults.

The bigger and more common problem is that the neighbouring houses settle slightly (not by much, say a millimetre or two) and then cracks develop, typically at abutments of walls and ceilings, and then of course the arguments and the rows and the financial pain starts.

The Party Wall Act can help to manage this situation by ensuring that surveyors monitor the works and agree an award detailing how damage will be made good and at whose cost, but ultimately the only solution is more care in building. And that costs money.

And as the enthusiasm for basements under terraced houses develops, so do the basement construction ‘specialists and experts’ who actually see the works as straightforward and a bit like a loft conversion only downwards… and as loft conversion work falls away because of the financial situation. lucrative basement construction seems like a good idea, especially if you can do it cheaper than others by cutting one or two little corners – like underpinning properly…

Or worse, there are the ‘do it yourself project managers’. Last year, we were called to a house where the neighbours had excavated their back garden for the basement. They hadn’t thought about building regulations or temporary support of the earth walls, let alone the health and safety of the eastern European labourers who were hand-digging the hole 3m down. A quick panic and emergency temporary support works and suddenly ‘do it yourself project management’ had cost them £25k – cheap when compared to a death arising as a result of negligent actions.

We would go so far as to say that every basement project where we have provided party wall services has ended up with damage being caused to next door, even where the schemes were properly designed, managed and constructed. The work is invasive and inevitably settlement happens. This BBC Inside Out extract shows the sort of damage that occurs.

In the worst case we were involved in, the basement constructor ended up paying £250,000 in fees and costs to rebuild his neighbours’ house because the engineering design was not properly executed.

Our advice to you

If you want to build a basement, follow these simple rules:

  • Make sure that you get decent party wall advice. Have it properly designed and remember, cheap is not best
  • Get decent insurance – your contractors insurance may well not be good enough and you (not the contractor) will be personally liable for the costs of any damage – although you may well be able to claim against them.
  • If you’re next to a basement construction – get decent party wall advice and make extra sure your neighbour is adequately insured.

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(Originally published by Steve Way, 26 February 2011)