I've done some conceptual planning along with a rough budget for the above Brethren suggestions for you.
First you will need to convert the 10'x10' space you are considering for the BBQ room into a small, efficient 3 BR house. Then you will, with very careful planning, probably be able to then convert the old house space into your BBQ suite without having to add any more square footage.
As far as cost, if you stick to the bare necessities suggested, you should be able to obtain beneficial occupancy for just under $425,000, which includes a small contingencpy.
Please let me know if I can be of any more assistance with this exciting project.