[kach22i]

Architect.

You could find a builder that typically works with an architect and get the design as a phase-1 of the contract. However such contracts typically lock you into using that builder so you cannot get outside bids.

By dealing with the architect first, the drawings can be bid on by any number of builders.

I've never seen an addition designed by an owner that added value to a home, the owner may have been happy with it but that was all.

There is some sad stuff out there, good intentions that lead to a disaster.