Hi

We're in the process of house hunting and have come across a nice
place with room for improvement.

The property is about 20 years old and has an attached double garage.
To make more upstairs space I would like to build on top of the
garage.
The owner reckons the existing garage foundations are good enough as
they are, but the garage is just double layer brickwork (no blockwork/
cavity).
In the upper storey access would be obtained by knocking through a
bathroom to make a short corridor into the new rooms.

I guess there are two ways to do it:
1) Increase foundation strength and put in blockwork.
2) Use an internal timber frame to support the additional weight of
the upper storey.

Assuming planning permission was granted (a risk to assume I know)
then I wondered on the approximate cost for this? Would anyone have
any idea how much to get the structural work done to a bare plaster
level?

Thanks
Marc