And make sure your builder does the usual checks on where any services (electricity, gas, water, bt etc etc) are... as well as checks on the level of the water table (which may rise and fall according to the rainfall levels).... How do you propose dealing with any outfalls from the basement (grey and foul water) - those need to fall away from the property or be pumped out - adding to the cost. The underground structure will need to be *tanked* ie waterproofed - another expense and the walls may have to act as retaining walls. Also the roof of this basement may need to be structurally capable of coping with another structure on top in the future. Don't forget to keep all calcs and findings too, in case you sell the land and property in the future - you may have to pass this information over to the new owners under the CDM Regulations.... Don't forget Fire Regs, Building Regs, and The Party Wall Act if you are close to a neighbouring property, etc etc...
I'm thinking it would probably be cheaper for you to simply move house...sorry !