Your Dimensions
(No, not those dimensions)

Try to be careful that you build your addition with proper
dimensions. Something like 8'X20' or 15'X30' is going to
give you a long narrow room.

You should build a room addition with some symmetry.
10'X12', 14'X16' is usually ideal because one dimension is
reasonably close to the other.

On one of my estimate calls the homeowner wanted to
build an 8'X30 rear addition. I said "Is this going to be a
bowling alley? He then ushered me out of the house
because he thought I was calling him stupid for wanting
to build such a monstrosity. He was right.

Here's a dimension tip that will save you money. Try to
build in 2' dimensions. )10'X12', 14'X16')
Most building materials come in 2' dimensions. This
saves the contractor plenty of labor.

It will actually cost you more to build a 11'X12' addition
than a 12'X12' room additionbecause of the increased labor.