Square footage matters, but access, structure, utilities, selections, and sequencing often explain more of the final number.
A budget is most useful when it shows the reasons behind the total. Labor to protect the house, remove existing work, move plumbing, alter framing, coordinate inspections, and restore adjacent finishes can be substantial before a decorative item is selected. A lower-cost faucet does not offset an unplanned drain relocation. Conversely, keeping a workable layout can preserve money for cabinetry, tile, lighting, or other elements the homeowner experiences every day.
Allowances should be specific enough to shop against. A single materials allowance hides which decisions carry risk. Separate allowances for cabinetry, counters, fixtures, tile, hardware, appliances, lighting, and glass make the estimate easier to evaluate. Once selections are made, each allowance should be reconciled against the actual order. This prevents an attractive preliminary total from depending on unrealistically low placeholders.
Contingency is different from contractor overhead and different from an undefined cushion. Existing homes can conceal damaged framing, obsolete wiring, unsupported modifications, or plumbing conditions that cannot be confirmed before surfaces open. The appropriate contingency depends on how invasive the work is and how much of the existing assembly can be inspected. The goal is not to predict every surprise. It is to decide in advance how the team will price, document, and approve concealed work if it appears.