Burrowing beneath
Burrowing beneath your house: how it is done
Any advice on digging out a basement should be introduced with a warning: do not try this at home - unlike the Islington man who recently had his house condemned after some DIY burrowing.
The essential element of the work is "underpinning" - extending the foundations downwards so that your house does not fall into the hole which is being created. If your house is not detached, it will first be necessary to gain an award under the Party Wall Act.
Until relatively recently, basements tended to be tanked - that is, lined with a layer of dense, impervious cement. This would hold back the damp for some time, but after a while, the tanking compound had a tendency to "blow" away from the wall.
Nowadays, basements tend to be lined with a plastic membrane. A secondary wall is then built inside the lining. Water that seeps in runs down the outside of the membrane and is collected in a sump beneath the floor, from which it is pumped away. So long as the pump continues to function, there is no reason why a modern basement should not be as dry as any other room in the house.
Before any of this can be attempted, it is vital to carry out a survey of everything that might run beneath your home by contacting all relevant electricity, gas and water companies. Sometimes services can be diverted - though perhaps not the Metropolitan line.