Start with the same scope
A low quotation may cover a smaller repair area, fewer details or less preparation. Give each contractor the same photographs, plans, leak history and access information. Ask each to identify the affected surfaces and explain whether the proposal treats a local defect or a larger waterproofing system.
Check what the price includes
Request measured quantities, substrate preparation, removal of failed materials, repairs, primer, membrane and protective finish where applicable. Include corners, upstands, penetrations, outlets and thresholds in the discussion. Ask whether access equipment, disposal, testing and reinstatement are included or separate.
Identify likely extras
Concealed damage, wet insulation, defective drainage and inaccessible areas can alter the work. Tile replacement, decorating the ceiling below, moving AC units and repairing plumbing may be separate items. Ask how additional work will be priced and approved, and how it affects the completion date.
Agree completion and aftercare
The proposal should explain verification, handover records, maintenance and written warranty terms. Compare the covered areas, exclusions and claim procedure. Before accepting a price per square metre, confirm that all quotations specify comparable preparation and system build-up.