The other thing to consider is the quality of the space. The rooms created would be just under 3m x 3m (approx 9ft x 9ft) which would be very tight for a double bedroom. So this means this space could be used for a small bedroom or perhaps bathroom or dressing room depending on how the internal layout turns out.
This idea was put forward by our first architect for consideration. The idea is to take the outline of the garage (original and new) on the east side and use this as a footprint to construct two new 'houses' to either side of the current house.