A home addition is really six or seven smaller projects that have to happen in a strict order. Understanding that order is the best way to judge an estimate, plan your budget, and know how long your household will be living around construction.
Everything starts with the design and permit process. Plans and permits have the longest lead time, often several weeks to a couple of months depending on your city, so they get started before any demo. Site prep comes next in the planning: the foundation can't be poured until plans are approved, and framing can't begin until the foundation has cured.
Demo and site preparation are relatively quick, usually a few days. Then the structural work: foundation, framing, roofing, and any changes to load-bearing walls that tie the new space into the existing home. Rough-in follows—plumbing, electrical, and HVAC—then insulation, drywall, and paint, because it's far easier to finish walls before trim and flooring go in.
Once the drywall is finished and painted, flooring goes in, doors and trim are set, and fixtures like lights, outlets, and plumbing fixtures are connected. Exterior siding, stucco, and paint tie the new structure into the old one, and a final inspection closes out the permit.
A good crew builds that sequence into the written estimate with dates attached. If you're comparing estimates, ask each contractor who handles the framing, drywall, paint, and flooring. The fewer subcontractors in the chain, the fewer gaps in the schedule.