Building on a Sloping Plot: A Worked Example of Earthworks and Levels

An illustrative calculation of cut, fill, retaining and foundations on a sloping site, and why a split-level design can turn the slope into revenue.

A slope is either a cost or an opportunity, and the difference is decided at concept design. Levelling the whole plot to one platform is the instinctive answer and often the expensive one, because the cost sits in disposal, imported fill, retaining and the foundations that then have to reach through the fill. This worked example prices both approaches on illustrative figures.

The situation

An owner has a 1,200 square metre plot, 40 metres deep and 30 metres wide, with a fall of 4 metres along its depth. The design team has proposed two approaches: a single level platform at mid-height with retaining to the lower boundary, or a split-level building stepping down the slope in two platforms. The owner wants to know what each costs before the layout is fixed.

Assumptions (illustrative)

• Plot 1,200 square metres, 40 metres by 30 metres, with a 4 metre fall along the 40 metre depth.
• Illustrative excavation and disposal: SAR 50 per cubic metre combined.
• Illustrative imported engineered fill, placed and compacted: SAR 75 per cubic metre.
• Illustrative reinforced retaining wall: SAR 950 per square metre of wall face.
• Illustrative extra cost per pad foundation reaching through fill: SAR 6,000.
• Illustrative construction rate SAR 2,400 per square metre and sale rate SAR 5,500 per square metre.

Step 1: Quantify the earthworks for a single platform

Setting the platform at mid-height means cutting the upper half and filling the lower half. Upper half area 600 square metres at an average depth of 1 metre gives 600 cubic metres of cut, and the lower half needs 600 cubic metres of fill.

A 4 metre fall does not mean a 4 metre excavation. Placing the platform at mid-height halves the depth at each end, which is why the level chosen for the ground floor is a cost decision rather than an architectural one.

Step 2: Price the cut and disposal

600 cubic metres at SAR 50 per cubic metre equals SAR 30,000.

This is the cheapest part of the exercise, which is why owners assume levelling is inexpensive. The cost is not in removing the material, it is in everything that follows from having a platform partly made of fill.

Step 3: Price the imported fill and the boundary retaining

Fill: 600 cubic metres at SAR 75 equals SAR 45,000. Retaining to the lower boundary: 30 metres wide at an average 2 metres high equals 60 square metres at SAR 950, which is SAR 57,000. Subtotal SAR 102,000.

Imported engineered fill costs more than removing spoil, and the retaining wall that holds it in place is a structure in its own right. Together they are three times the cost of the excavation everybody focused on.

Step 4: Add the foundation consequence of building over fill

Eight pad foundations on the filled half must reach competent ground below the fill: 8 at SAR 6,000 extra equals SAR 48,000. Total for the single platform approach: 30,000 plus 102,000 plus 48,000 equals SAR 180,000.

You cannot found a building on fill you have just placed without either deepening the foundations or improving the ground to an engineered standard. This is the line most often missing from an early estimate of levelling a site.

Step 5: Price the split-level alternative

Two platforms stepping 2 metres: cut 250 cubic metres at SAR 50 equals 12,500; fill 250 at SAR 75 equals 18,750; internal retaining 30 square metres at SAR 950 equals 28,500; reduced boundary retaining 30 square metres equals 28,500; extra structural cost for the stepped slab and internal stairs SAR 65,000. Total SAR 153,250.

The split level saves SAR 26,750 on these assumptions, which is real but modest. Taken alone it is not a decisive argument, and that is why the comparison is usually settled on the wrong basis.

Step 6: Value the extra level the slope allows

The step creates a usable lower level of 120 square metres. Construction at SAR 2,400 equals 288,000, against sale value at SAR 5,500 equals 660,000, a net gain of SAR 372,000.

This is where the decision is actually made. The slope, used rather than removed, produces lettable or saleable area that a flat platform buries under fill. The earthworks saving is incidental next to the revenue the level generates.

What changes the answer

• Whether the lower level qualifies as permitted floor area under the municipal conditions, which must be confirmed.
• The bearing capacity below the fill, since deeper foundations can cost far more than assumed here.
• Rock in the cut, which can multiply the excavation rate.
• Whether excavated material is reusable, which removes or doubles the fill line.
• The achievable sale or rent rate for a lower ground level, which is often below the rate for upper floors.

What the numbers tell you

• The platform level is a commercial decision and belongs in the concept design review.
• Excavation is cheap; imported fill, retaining and deeper foundations are not.
• Never found a building on newly placed fill without pricing the foundation consequence.
• A slope can generate saleable area, and that value usually exceeds any earthworks saving.
• Confirm with the municipality how a lower level counts towards permitted area before relying on it.

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.

Domira designs, builds and manages projects across Saudi Arabia for owners who are based abroad: land appraisal, feasibility, design, construction, handover and asset management under one accountable contract. Send us the plot or the brief and Domira will put the numbers in front of you before you commit.