Unwin's pamphlet set out two ways of developing the same ten acres - by-law terraces at roughly thirty to forty houses per acre against garden-city layout at about twelve - and costed both. Because the tighter scheme spends far more per house on road frontage, kerb and drainage, he argued the low-density plan cost the landowner no more while giving each family a garden, light and air. The effect was to turn density from a moral complaint about slums into a number that could be specified, compared and enforced: houses or persons per acre, later dwellings per hectare, plot ratio and floor space index. British housing policy took up twelve houses per acre as the working standard, and Unwin carried it into the Tudor Walters Report of 1918.
It fixed density as the central metric of modern planning, and fixed it low - the arithmetic behind interwar council estates, green-belt suburbia and the sprawl later planners spent decades arguing back. Every density debate since, from the compact city to floor-area-ratio bonuses, is conducted in the units Unwin popularised.
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