Help & Advice
When your budget is tight, building into the ground, with its cost uncertainty might mean that the prudential builder should forego the benefits a basement normally bestows. Even a full geological site survey with several test holes can only ever paint a partial picture of what you may discover when you start digging. We have seen unknown underground streams, undocumented service pipes, archaeological items. the presence of which are felt extra keenly when you budget is tight.
If you have close neighbours then there is a risk to the neighbouring structure from your hole in the ground. To stop this then a piling solution may be required to hold back your neighbours home from falling into your hole. The general rule is that the load from a structure spreads out at 45 degrees from the bottom corners of the foundation and you must not cut through this imaginary line. See the drawing at the side for a sketch showing this load line.+
If you have a high water table on your site then a basement is not impossible but it can add to the cost. This increase in cost is not only for extra costs in waterproofing so much as the possible need to have to “de-water” the site during the construction. Such de-watering adds both time and cost to the build budget.
If you have poor access then extra costs might be incurred through movement of materials and also for removal of spoil from the dig-out.
If you are building an extension to an existing property, then a basement may seem like a good idea. It rarely is as this will need underpinning under the existing house and underpinning can be very expensive. You will need to underpin the existing structure to the length of the extension plus at least an extra metre to either side. For underpinning costs allow £1500 to £2000 per linear metre of underpinning.
Sandy soils may look ideal for basements as these are usually well drained. However, when the sand has little cohesion and is described as “running sand” then the dig can take three times longer than a stiff clay soil and may require “battering back” to stop spillage.
However, if the above does not deter you then go for it.