How to estimate a building cost before construction starts
A plain-language route from floor area and finish level to a reviewable planning range—and the limits you should understand.
Section 01
Start with the information you actually know
An early estimate does not need to pretend that a full drawing and bill of quantities already exist. Start with the proposed location, approximate floor area, number of floors, room mix and expected finish level.
The result should be shown as a range. A single precise-looking number at this stage can hide uncertainty in the design, market prices and site conditions.
Section 02
Separate the main cost sections
A useful building estimate separates foundations, frame, walls, roof, openings, finishes, services, external works and preliminaries. This makes the figure easier to review and later replace with measured quantities.
- •Confirm whether the floor area is per floor or total for all floors.
- •Treat external works and professional fees separately.
- •Record every major assumption beside the estimate.
Section 03
Know when to move to a BOQ
Once drawings and specifications are available, the planning range should be developed into measured items with descriptions, units, quantities and rates. That is the stage where a quantity surveyor or experienced estimator adds the most value.