Cost of a basement extention

[Yellowduck]

I would be grateful for any rough prices....

I am looking to build a 4m extention to a semi bungalow, right accross the back of the bungalow, part pitch tiled roof over half the extension with vaulted ceiling over kitchen diner part. Other half which is ajoining neighbours side, will be bedroom will have flat roof with glass skylights.

The site is on a fairly hefty slope, so at the point the extension ends, the floor (if kept level with the existing bungalow) will be about 1m above ground level. I know a rough cost for an extention is about 1k per SqM, so looking at about £25k - £30k for the above.

However, thinking outside the box, it may be possible to dig out and create a basement room (same size 7m x 4m) under the extension and turn into a very small bedsit for son and his girlfriend.

Can anyone give me a very rough idea of what impact this would have on the cost, as there would obviously have to be tanking etc. There is full access for a dumper and a full size JCB if this would recduce the cost of digging out!

Only looking at a rough idea of the additional cost this may add to the original £25-30K.

Many thanks

Martin

 

[Blagard]

It is not as simple as you might think. With the room below the same size as the extension that is likely to have a major impact on the existing wall foundations which would need to lowered possibly including sections of the foundations to the party wall and other return wall. Would your neighbour agree?

Until an investigation (trial pit) is done to establish what your existing foundations are, that alone is likely to have significant cost implications.

Although you are on sloping ground, ground water may also be a significant factor as even the low side of the basement is going to be quite deep. The nature of the tanking required will depend on those ground conditions as building the reverse of a swimming pool would be very expensive.

Other problems may be diverting drainage and other in ground services