Two Structural Options, One Building: A Worked Cost Comparison

The same villa on two different structural grids. A calculation showing why the decision belongs at concept stage.

Structural decisions are made quickly at concept stage and are almost impossible to revisit later. This example compares two grids for the same building to show how large the difference is, and why it deserves more attention than it usually receives.

The situation

A two-storey villa of 420 square metres is designed with an open plan reception requiring long spans. The structural engineer offers two options: a regular grid with an intermediate column, and a long span solution that keeps the space entirely open.

Assumptions (illustrative)

• Built-up area: 420 square metres.
• Option A: regular grid with an intermediate column, maximum span 6 metres.
• Option B: long span, maximum span 9 metres, no intermediate column.
• Assumed concrete volume difference: 18 cubic metres additional for Option B.
• Assumed concrete cost: SAR 950 per cubic metre in place.
• Assumed reinforcement difference: 4.5 tonnes additional for Option B.
• Assumed reinforcement cost: SAR 3,800 per tonne in place.
• Assumed additional formwork and labour for deeper beams: SAR 18,000.

Step 1: Additional concrete

18 cubic metres multiplied by SAR 950 equals SAR 17,100.

Longer spans require deeper sections, and depth consumes volume across the whole element rather than only at the span.

Step 2: Additional reinforcement

4.5 tonnes multiplied by SAR 3,800 equals SAR 17,100.

Reinforcement rises faster than span, which is why the cost of open space is not linear with the distance gained.

Step 3: Formwork and labour

Assumed additional SAR 18,000 for deeper beams and associated support.

Deeper sections need more support and longer cycle times, which affects both cost and programme.

Step 4: Total structural premium

SAR 17,100 plus SAR 17,100 plus SAR 18,000 equals SAR 52,200.

This is the price of removing one column. Whether it is worth paying is a genuine question with a defensible answer in both directions.

Step 5: Expressed per square metre

SAR 52,200 divided by 420 square metres equals about SAR 124 per square metre.

Expressed this way, it invites the right comparison: is an uninterrupted reception worth roughly the same as upgrading the floor finish throughout the house?

Step 6: The same decision after construction begins

Changing the grid after foundations are poured means redesign, demolition and rebuilding, at a multiple of this figure.

The comparison above is available only at concept stage. Afterwards, the question is no longer which option is better but what the change would cost.

What changes the answer

• Steel price movements affect the reinforcement component directly and can shift the premium noticeably.
• A post-tensioned or alternative system may change the economics for longer spans.
• If the deeper beams reduce ceiling height, the trade-off involves spatial quality as well as cost.
• Programme impact from longer cycle times adds preliminaries that this calculation does not include.

What the numbers tell you

• Ask for two structural options with costed differences at concept stage, before the grid is fixed.
• Express the premium per square metre so it can be compared with other uses of the same money.
• Consider the effect on ceiling height and services routing, not only the cost.
• Recognise that this is a decision with a deadline: it disappears once foundations are cast.

Limits of this example

Every figure above is an illustrative assumption used to show the method, not a quote, valuation or market rate. Costs, rates, fees and regulations change and must be confirmed with the relevant authority, lender or supplier before you commit.

If you are preparing to build, Domira can review your drawings, scope and contractor offers before you commit, and supervise delivery against the programme and specification you agreed.