Extension cost planning: build a budget before comparing builders

State what the area figure means

Explain the measurement basis and whether the estimate concerns shell construction or a finished space. Internal clear dimensions, external footprint and gross floor area are different quantities. A simple outline does not reveal ground conditions, drainage constraints, load paths or access difficulty. Keep those uncertainties visible rather than converting every area figure into a fixed total.

Budget for information and decisions

List design, measured surveys, structural input and relevant authority or professional fees as separate workstreams. Ask the professionals involved to identify what information is needed before a builder can give a firmer scope. A concept layout or survey-derived model helps discussion but does not establish permission, compliance or construction suitability.

Break construction into clear packages

Organise enquiries around enabling work, groundworks, structure, roof, openings, services, insulation, internal finishes and external making good. Identify what is measured, what is a design assumption and what remains a provisional allowance. Use the same drawings and specification revision for each builder so price differences can be explained.

Record site constraints and interfaces

Discuss access, material storage, occupied areas, waste, existing services and the connection between old and new work. Ask how discoveries that affect the scope will be evidenced and authorised. The programme should identify decision dates and product lead times, not only a proposed completion date.

Keep contingency distinct from agreed work

Show the base quote, provisional items, client purchases, fees and a separately chosen contingency. Do not count an allowance twice or spend contingency without a recorded decision. Check payment stages, exclusions, VAT and the method for agreeing variations. Keep the final scope and handover requirements with the project record for later reference.

Your quote checklist

• Area basis and design revision
• Survey/design/professional fees
• Ground and existing-service assumptions
• Shell versus complete finish
• Provisional items and client purchases
• Payment stages, variations and contingency