It's very important to have as much detail as possible written down in contract as regards what the builder's quotation includes - who pays for bathroom suites, doors, handles, skirting, light fittings, vent for dryer & extractor, whose responsible for filling small holes before painting - some builders pass this off as painter's job etc etc
Not every plumber is qualified to fit gas - make sure your builder is responsible for getting this done
Pre-finished doors and skirting are worthwhile