What drives the cost of a home addition?

Start with the footprint and the room program

Record approximate area, number of levels, intended rooms and the existing rooms affected. A bedroom wing, kitchen extension and bathroom addition are different scopes even if their floor areas match. Wet rooms, built-in storage and equipment selections belong in the brief before comparing estimates.

Ask the estimator which area definition is being used. Covered outdoor space, conditioned rooms and interior alterations should be identifiable rather than blended into one area figure. A price divided by floor area is a summary of a scope, not an explanation of it.

Separate the main cost groups

Use a worksheet with a description, pricing basis, included quantity or allowance, exclusions and remaining questions for each group. Identify whether design and approval costs are in the construction proposal or paid separately.

• Site preparation: access, removals, grading, utility conflicts and restoration.
• Structure and enclosure: foundations, framing, roof, exterior walls, windows and doors.
• Connections: openings into the house, roof transitions, floor levels and weatherproofing.
• Systems: plumbing, electrical, heating and cooling assessment, and distribution.
• Interiors: finishes, cabinetry, fixtures, trim and work in adjoining rooms.

Make the existing-house work visible

A proposal may show the new room clearly while leaving the old-house repairs vague. Ask how far flooring, ceiling repairs and painting extend. Specify the expected outcome when an exact material match is unavailable. Include removal and reinstatement of affected patios, fences, irrigation and landscaping where relevant.

For concealed conditions, request an investigation plan and a documented method for pricing discoveries. Ask what can be checked before signing and what remains inaccessible until construction. A reserve should respond to those unknowns; a single universal percentage cannot describe every property.

Compare a baseline and deliberate options

Keep a core scope that meets the room brief, then price optional finish or layout choices separately. This makes a meaningful decision possible without quietly removing necessary connections or weather protection.

The FTC recommends obtaining written estimates and understanding scope and materials. Apply that principle to a property-specific addition worksheet. Request local, dated quotations rather than treating online examples as local prices. Keep household costs such as temporary accommodation on a separate owner line so they are counted without being mistaken for the contractor’s work.