IMO, it's a little premature to be getting contractor quotes until you've decided how you want everything laid out IN DETAIL. I chose to "make do" with my sort of inadequate darkroom until we decided to have a contractor come in for a whole-house re-model. By then I had my darkroom plan laid out with pencil & paper on gridded paper (scale = one inch per line). I did several views as necessary - floor plan, elevation for each wall, ceiling. Darkrooms are three-dimensional spaces, of course. Important to show where electrical outlets are needed or may be needed in future (e.g. walls/ceiling), as well as the currrent draw required for each one (never have enough outlets). I found it very useful to make max dimension paper cutouts of each significant piece of equipment that can be moved around on the gridded paper for adequate placement and spacing.
Other thoughts that come to mind:
A search of the forum for threads on darkroom design will provide much food for thought.
Plumbing - behind the walls vs. accessibility for repairs, supply/drains.
Flooring - it seems spills are inevitable - access to ALL floor space (under everything) for mopping up is good.
Carpets have no place in a darkroom - trap dust and hinder mopping up.
Dust control and ventilation are crucial - exhaust fan to outside & filtered incoming air (some prefer positive pressure approach).
Lighting for placement of safelights, room lights, wet print viewing light(s), etc. (beware of fluorescents).
Anti-fatigue mats for long periods of standing + even a folding chair to sit occasionally for knees/back relief.
Ceiling height check - for max enlarger height, space for wall-hung cabinets, shelves, etc.
Darkened partitions around the enlarger to minimize reflections that can fog paper in easel.
How to best block light leaks around doors/windows.
Color of walls (wide variety of opinions and preferences on this).
In my case, the contractor found it very easy to make an accurate quote, as the details were already worked out. Plus he was able to suggest minor improvements possible with the special tools he had to work with.