Builders and architects will often use a rough $/m² figure when arriving at a preliminary estimate of the cost of building a home, making reference to tables of figures published by quantity surveyors or found online.
A typical example of the kind of cost per area table of used in cost estimates. The ‘Low’ figures represent a no-frills, volume-built house with the cheapest finishes and fixtures, and the ‘High’ figures indicate a custom, architect-designed house with high-quality fixtures and finishes.
Something that is not factored into these estimates, however, is the effect of the shape of a house on its cost: in other words, the effect that the floor-area-to-wall-length ratio has on the quantity of various materials required to construct a house of any given floor area.
Take the following simplified examples, which consider only the house in plan view, and assume equal wall heights.
A house that is a 10m x 10m square in plan has a floor area of 100m²:
Likewise, a house that is a 20m x 5m rectangle in plan also has a floor area of 100m²:
But the total length of external wall in the former is 40m, and that in the latter is 50m, representing an increase of 25%. This means that the rectangular house shown above requires 25% more studs, plates and noggings, 25% more interior and exterior cladding, 25% more insulation, 25% more building wrap, and 25% more skirtings and cornices than the square house.
A courtyard house, with narrow internal corridors connecting the two main areas, decreases the ratio even further:
There is also the effect of articulation to consider, i.e. adding ‘ins and outs’ to the exterior wall:
At the other end of the spectrum, the absolute most efficient shape for a building if your only aim is to maximise the floor-to-wall ratio is the perfect circle, but this shape is impractical from both a construction and an interior layout/planning point of view:
So why would anyone build anything other than square? In fact, most modern volume-built homes are essentially square in plan, or at least fatter than they are skinny, with a central corridor serving rooms on either side. This allows developers to maximise the floor area of these homes on their relatively small lots while keeping the cost of constructing the envelope of the house relatively low.
A ‘fat’ plan typical of most volume-built houses.
But there are several advantages to opting for a narrow plan over a square one. Cross-ventilation and the penetration of natural light into rooms are both optimised, and there are more opportunities for northern (or, if you are in the northern hemisphere, southern) exposure; in the extreme scenario, every room can be given northern exposure, with a corridor running the full length of the southern side of the house.
Of course, on most projects the advantages of narrow plans listed above must be weighed against budgetary and site constraints and other considerations, but it doesn’t hurt to at least have floor-to-wall ratio in mind when determining the best plan-form for a house. Construction is a one-time cost, but the utility and amenity of a home are lifelong.