Create space for a bathroom: Extend a room over the stairs or something else?

oxfordmark

Hey

We are in the process of buying a house, so we are looking at plans.

A house on another street is exactly the same floor plan as ours.

What they have done is decreased the size of bedroom 1 and created a new room. The floor of the new room has been extended about 2-3 feet (guess) over the stairs, this has allowed them to put in a small bedroom. We were thinking about doing the same but installing a bathroom instead.

The property we are buying:https://docs.google.com/file/d/0Bx64yNpWwKBWRElCT2Yxc0VpNjg/edit?usp=sharing


The house that has done the work:
https://docs.google.com/file/d/0Bx64yNpWwKBWd0pyaHZSQjBJMjg/edit?usp=sharing

How much would we be looking at paying for getting someone to do all the building work?

Looking at the plans of our house, can you see any other way of keeping 3 bedrooms (obv smaller) but making space for a bathroom?

Thanks
Thanks