Cost estimating by the designer

The stages of estimating

Concept level

From unit rates: floor area, building type, quality level. This gives an order of magnitude, not a price. The margin of error is significant.

Permit level

The dimensions, the structural system and the main build-ups are known. The estimate is more accurate, but the details are still open.

Detailed design level

An itemised bill based on quantities. This is what comparable tenders can be sought on. The content of the detailed design.

The paradox

The biggest cost decisions — massing, spans, structural system, quality level — are made at the concept stage, when the estimate is least accurate.

So cost-conscious design does not start at the detailed design but at the beginning: that is where the consequences of the decisions have to be visible, at least in order of magnitude.

What moves the cost

The geometry

Complex massing, many corners, many different spans — all of them add cost.

The spans

Half a metre less span often means a whole section size smaller, throughout the building.

The quality level

Finishes, services, sound insulation expectations.

How well the structure is optimised

On the material cost of the structure itself, roughly 40% can be saved with a well-calculated, optimised design compared with an over-sized one. For the other disciplines this cannot be measured as precisely, but a more accurate structure is easier and faster to work to.

The foundations and the ground

Often the largest uncertainty in the estimate.

What an estimate does not contain

• the contractor’s risk and margin,
• market movement,
• a contingency for unforeseen items,
• permitting and utility costs, unless separately named.

These have to be handled separately, or the estimate and reality part company.

The point

Cost is decided by the concept, not by the detailed design. The accuracy of the estimate grows later, while the ability to influence it shrinks.