In some areas of the country (USA) if you leave 1 wall and part of the
original floor in place, you can build a much larger structure on a
remodeling permit. If you don't leave the wall, you must get a new
construction permit. Depending on where you are, the difference
can be many thousands of dollars in permit costs and land use
As for a materials estimate, you need to get a set of prints together
and take them to a local lumber yard or construction supply yard.
Putting together an estimate like yours is a lot of work...but they
will be happy to do it to get your business.