Separate cost from timing
A total development cost does not show when cash is required. Land, design, approvals, construction, financing, marketing and taxes occur at different times, so cash-flow timing can materially change funding needs.
Make assumptions visible
Sales pace, rental levels, build cost, interest rate and programme should be shown as assumptions that can be tested. A model becomes misleading when uncertain inputs are presented as fixed outcomes.
Use contingencies for uncertainty, not optimism
Contingency should reflect what is not yet known and should reduce as information improves. It is not a substitute for proper scope definition or a reason to ignore known cost.