Building a 12 x 12 bedroom

The first think would be to check with your city/county "building" and "zoning" department(s).

You need to find out what the setback, lot size, building size, lot coverage, etc reuirements are from the zoning department.

And from the building department what building codes are used, if you need to have plans done by an architect and/or engineer. If you are in an area that requires special designs work (earthquake, hurricane, high winds).

Check the C&R's and homeowners association rules for any restrictions on type of materials or architectural review requirments.

Also check on the HVAC and electrical to see if they can support the additional space.