Why a Duplex BoQ Is Fundamentally More Complex Than a Bungalow BoQ
A duplex (two-storey house) introduces several additional cost elements that a bungalow simply does not have: a suspended reinforced concrete first-floor slab, a staircase, doubled wall heights, significantly more structural steel and concrete, and a roof that covers only the upper floor rather than the entire building footprint. These differences mean a duplex Bill of Quantities is typically 40 to 60 percent more expensive per square metre than a bungalow of equivalent total floor area — and contains 20 to 40 more line items that require specialist Quantity Surveyor knowledge to quantify correctly.
In 2026, a Bill of Quantities for a standard 4-bedroom duplex in Nigeria has a total construction value of ₦55 million to ₦100 million depending on location, soil conditions, and finish specification. This guide walks through every section of a duplex BoQ so you understand exactly what drives the cost and where you can exercise control.
How a Duplex BoQ Differs from a Bungalow BoQ
| Cost Element | 3-Bed Bungalow | 4-Bed Duplex |
|---|---|---|
| Foundation type (typical) | Strip or pad | Raft or deepened strip |
| External block count | 2,500 – 4,000 | 5,500 – 9,000 |
| Suspended first-floor slab | None | ₦3.5M – ₦8M |
| Staircase | None | ₦600,000 – ₦2,000,000 |
| RC columns | Minimal (300m² plan) | Significant (higher loads) |
| Roof area vs floor area | Roof covers full plan | Roof covers upper floor only (50%) |
| Total BoQ sections | 8 – 10 | 10 – 12 |
| Total BoQ line items | 80 – 120 | 120 – 160 |
Section 1 — Substructure and Foundation
A duplex imposes significantly higher structural loads than a bungalow — roughly double the wall weight and a suspended slab that transfers live and dead loads through columns to the foundation. Most Nigerian structural engineers specify a raft foundation for duplexes, particularly on the mixed soils common in Lagos, Port Harcourt, and coastal areas. On firm, well-drained laterite subsoil in Abuja, Ibadan, and Enugu, a deepened strip foundation with wider widths may be adequate, but this must be confirmed by a soil investigation report before any excavation begins.
| Item | Unit | Qty | Rate (₦) | Amount (₦) |
|---|---|---|---|---|
| Excavation to formation level | m³ | 55 | 9,000 | 495,000 |
| Hardcore filling and compaction (150mm) | m³ | 40 | 12,000 | 480,000 |
| Blinding concrete (1:3:6), 75mm | m² | 110 | 4,500 | 495,000 |
| Raft foundation slab (1:2:4) with Y16 BRC mesh | m³ | 28 | 145,000 | 4,060,000 |
| Starter columns from raft to DPC level | m³ | 3.5 | 130,000 | 455,000 |
| Damp-proof membrane (1000 gauge polythene) | m² | 110 | 1,800 | 198,000 |
Section 1 subtotal: ₦6,183,000
Section 2 — Ground Floor Superstructure
The ground floor block work, columns, and ring beams must be designed to carry the combined weight of the ground floor structure, the first-floor slab, the first-floor walls, and the roof. This is why ground-floor column sizes and reinforcement quantities are significantly larger in a duplex than in a bungalow.
| Item | Unit | Qty | Rate (₦) | Amount (₦) |
|---|---|---|---|---|
| 9-inch hollow sandcrete blocks (ground floor external) | No. | 2,800 | 1,100 | 3,080,000 |
| RC columns 300x300mm (ground floor), 1:2:4 | m³ | 7.5 | 130,000 | 975,000 |
| Y12 and Y16 reinforcement bars — columns | tonne | 1.8 | 680,000 | 1,224,000 |
| Ground floor ring beams and lintels (1:2:4) | m³ | 5.5 | 120,000 | 660,000 |
| Formwork to columns and beams | m² | 85 | 6,500 | 552,500 |
Section 2 subtotal: ₦6,491,500
Section 3 — Suspended First-Floor Slab
The suspended first-floor slab is the most technically demanding and most expensive per-m² element of any duplex. It is typically a 150mm to 175mm reinforced concrete slab, cast on removable timber or metal formwork (props and plywood decking), which is struck after the concrete has achieved sufficient strength — usually 14 to 21 days after casting. In Nigeria, the all-in cost for a suspended slab (concrete, reinforcement, formwork, and labour) runs ₦35,000 to ₦75,000 per m², depending on the specification and the city. Never allow a contractor to proceed with first-floor slab casting without a structural engineer's supervision and a proper curing regime.
| Item | Unit | Qty | Rate (₦) | Amount (₦) |
|---|---|---|---|---|
| Formwork to soffit of suspended slab — erect and strike | m² | 105 | 12,000 | 1,260,000 |
| Y12 BRC mesh reinforcement (bottom layer) | m² | 105 | 7,500 | 787,500 |
| Y10 secondary reinforcement (top layer) | m² | 105 | 4,500 | 472,500 |
| RC suspended slab (1:2:4) 175mm thick | m³ | 18.4 | 95,000 | 1,748,000 |
| Curing compound and wet curing (14 days) | m² | 105 | 1,200 | 126,000 |
Section 3 subtotal: ₦4,394,000
Section 4 — First Floor Superstructure
The first-floor block work, columns, and roof-level ring beams repeat many of the ground-floor items. However, there are additional complexities: the staircase opening must be clearly detailed and framed in the BoQ; balcony parapets require additional block courses and coping; and the roof-level ring beam carries greater combined loads from the roof structure and must be sized accordingly by the structural engineer.
| Item | Unit | Qty | Rate (₦) | Amount (₦) |
|---|---|---|---|---|
| 9-inch hollow sandcrete blocks (first floor external) | No. | 2,500 | 1,100 | 2,750,000 |
| 6-inch blocks — internal partitions (first floor) | No. | 600 | 780 | 468,000 |
| RC columns 250x250mm (first floor) | m³ | 5.5 | 125,000 | 687,500 |
| Roof-level ring beam — heavy section (1:2:4) | m³ | 4.8 | 120,000 | 576,000 |
| Parapet block walls (balconies) | No. | 300 | 1,000 | 300,000 |
Section 4 subtotal: ₦4,781,500
Section 5 — Staircase
The staircase is one of the most consistently underestimated items in Nigerian duplex BoQs. A cast in-situ reinforced concrete staircase with ceramic tile finish typically costs ₦600,000 to ₦1,200,000 for a straight flight with mild steel balustrades. A feature staircase with a quarter-turn or half-turn landing, stainless steel or wrought iron balustrades, a hardwood handrail, and granite or Italian marble treads can easily reach ₦2,000,000 to ₦4,500,000. The staircase must always be a separate BoQ section — contractors who bury it in the "superstructure" lump sum are making it impossible to check or verify.
| Item | Unit | Qty | Rate (₦) | Amount (₦) |
|---|---|---|---|---|
| RC staircase slab and steps — formwork and concrete | m³ | 3.5 | 140,000 | 490,000 |
| Y10 reinforcement to stair slab | kg | 180 | 900 | 162,000 |
| Ceramic tiling to treads and risers | m² | 18 | 14,000 | 252,000 |
| Mild steel balustrade — supply and fix | m | 12 | 22,000 | 264,000 |
| Hardwood handrail (65mm diameter) | m | 12 | 8,500 | 102,000 |
Section 5 subtotal: ₦1,270,000
Sections 6 to 10 — Summary
| Section | Typical Cost (Standard, Lagos 2026) |
|---|---|
| Roofing (trusses, covering, gutters) | ₦2,800,000 – ₦6,000,000 |
| Joinery (doors, windows, railings, balustrades) | ₦3,000,000 – ₦6,500,000 |
| Internal finishes (plaster, screed, tiles, paint) | ₦6,500,000 – ₦14,000,000 |
| External finishes (render, external paint) | ₦1,500,000 – ₦3,000,000 |
| Plumbing (3 bathrooms, kitchen, overhead tank) | ₦2,500,000 – ₦5,500,000 |
| Electrical (2 floors, gatehouse, security lights) | ₦2,200,000 – ₦4,500,000 |
| External works (fence, gate, paving, septic, borehole) | ₦3,500,000 – ₦9,000,000 |
Total Duplex BoQ by City — Standard Finish (2026)
| City | Economy (₦) | Standard (₦) | Luxury (₦) |
|---|---|---|---|
| Lagos | 56M – 68M | 72M – 98M | 105M+ |
| Abuja | 48M – 60M | 62M – 88M | 95M+ |
| Port Harcourt | 46M – 58M | 60M – 84M | 90M+ |
| Ibadan | 40M – 52M | 52M – 74M | 80M+ |
| Enugu | 38M – 50M | 50M – 70M | 75M+ |
All figures exclude land cost, professional fees (8 to 12 percent of construction value), and statutory approvals. Always add a 15 percent contingency to your total BoQ value before agreeing a final contract sum with any contractor. The contingency is not negotiable — it exists to protect you from price rises, unforeseen site conditions, and design changes during construction.
Key Things to Watch in a Duplex BoQ
- Suspended slab specification: Ensure the BoQ clearly states the slab thickness (minimum 150mm for residential), reinforcement specification (mesh size and bar diameter), and curing method. Under-specified slabs are a serious structural and safety risk.
- Staircase pricing: If the staircase is not a separate BoQ section, ask the contractor to break it out before signing the contract. You need to know what you are paying for this element.
- Column sizing consistency: The structural engineer's drawings and the BoQ must agree on column sizes and reinforcement. A BoQ that shows 250x250mm columns but the drawings show 300x300mm means the contractor has underpriced the structural frame.
- First-floor blockwork separate from ground floor: Some contractors total all blockwork in a single line. Insist on separate pricing for ground floor and first floor — materials are delivered at different times and the quantities can be verified against the drawings independently.
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