If you have ever asked a builder "how much to put up 4 units of 2 bedroom flat" and gotten back a number that felt suspiciously like "one unit's cost, times four" — that number is probably wrong. Not because the builder can't do maths, but because a block of flats genuinely does not cost four times what one standalone unit costs. Here is what actually changes, and what a proper unit-based estimate needs to account for.
What "2 Bedroom Flat" Actually Means
Worth stating plainly, because it trips people up: in Nigeria, "2 bedroom flat" means 2 bedrooms plus a living/sitting room — the living room is implied, not something you have to ask for separately. The same goes for "3 bedroom flat" and up. A costing that treats "2 bedroom" as literally just two rooms and nothing else is undercounting before it has even started.
A typical unit, then, is: 2 (or however many) bedrooms, 1 living room, 1 kitchen, and 1-2 toilets/bathrooms — and a "block of flats" is simply several of these independent units stacked or arranged side by side, each with its own full set of rooms, sharing a footprint and some structure but functioning as separate households.
Why "One Unit × Number of Units" Falls Short
The naive approach treats a block of flats as pure repetition: cost one 2-bedroom unit properly, multiply by 4, done. It feels reasonable, but it silently drops several costs that a standalone unit never has to pay because it doesn't share a building with anyone:
- Party walls. Every pair of neighbouring units needs a solid separating wall between them. A single standalone unit has none of these — all its walls are either external or internal-to-itself. Multiply a standalone unit's cost by 4 and you've multiplied its internal wall ratio too, but you still haven't added the walls that exist because units are next to each other.
- Fixture counts that don't scale the way people expect. Plumbing and electrical fixtures (WC, wash basins, kitchen sinks, sockets, light points) genuinely do scale per unit — 4 units need roughly 4× the fixtures of one. This part naive multiplication actually gets right. The mistake is assuming everything scales this cleanly, when party walls and shared structure don't.
- Shared foundation and roof economies. Four units built as one contiguous block share one foundation run and one roof, both of which are driven by the building's combined perimeter — and a combined footprint's perimeter grows slower than 4× as you add units, because shared edges between neighbouring units aren't re-walled or re-roofed. This one actually works in your favour: it's part of why a real block of flats typically costs less than 4 completely separate detached units would, even after adding the party walls above.
Put together: naive multiplication overstates some things (treating shared foundation/roof as if it were 4 separate ones) and understates others (missing party walls entirely). The two errors partly cancel out, which is exactly why the naive number can look "close enough" while still being wrong in ways that matter once you're actually pricing a contract.
A Worked Example
Take a single-storey block of 4 units of 2-bedroom flats — each unit: 2 bedrooms, 1 living room, 1 kitchen, 1 toilet, 1 ensuite. Run one unit through a proper cost engine on its own, and you get a certain total. Multiply that by 4, and you get one number. Now model all 4 units as one combined building — same room program per unit, but sized and walled as a single contiguous structure — and you get a different number, because:
- The combined footprint's perimeter (and therefore external walls, foundation, and roof) is noticeably less than 4× the perimeter of one isolated unit, since neighbouring units no longer need separate external walls where they meet.
- A real party-wall cost gets added back in — full masonry between each pair of neighbouring units, something a single unit's costing never included.
- Fixture counts (WC, sinks, sockets, light points) scale up cleanly by unit count, same as the naive approach assumed.
The net effect in practice: the properly-modelled block usually lands close to the naive 4× figure in total, but the money is in different places — less on external walls/roof than naive multiplication assumes, with a real, separate line item for party walls that a lot of manual estimates simply never include at all. If your estimate has no party-wall line item anywhere, that's worth double-checking before you sign a contract based on it.
The Practical Takeaway
If you're getting a block of flats quoted, ask specifically whether the quote accounts for party/separating walls between units as their own item — a surprising number of manual estimates don't, because it's easy to think of "cost per unit" and forget that units next to each other need a wall between them that a standalone house never pays for. And if you're doing the maths yourself, resist the shortcut of pricing one unit and multiplying — describe the actual building (how many units, how they're arranged across floors) and price the whole structure as one, not as several unrelated houses that happen to be near each other.
Costing a Block of Flats? Do It Unit by Unit
The Professional Estimator now has a dedicated Number of Units option for Block of Flats — describe one typical unit's rooms, say how many sit on each floor, and it multiplies the room count, adds the party walls between units, and scales fixture counts automatically.
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