In the real world of residential construction, you don't write specs. The exception to that (sometimes) is homes that cost several million dollars. If you have a good architect, they should include all the specs that you want on your plans (furnace, AC, electrical plan, interior elevations for the trim guy, etc.).
Otherwise, a simple scope of work for each trade to base their bid on isn't a bad idea. Even better, let the contractors bid the house based on your guidelines and ask that they list out exactly what is being included in that bid. Don't forget to have the archtect reference the applicable code on the plans so you're sure that they're at least bidding work compliant with minimum requirements.