building a basement - anyone done it, costs etc

[wombleprincess]

we're thinking of building a basement and single story rather than double story extension? anyone have any experience of this, costs etc?

my sister hd it done - no plumbing (adds to cost) just two large rooms and some storage and I think it was £100-£150k. Much more expensive than a double storey extension and digging the foundations out is a nightmare.