This article investigates how speculative house-builders determine the speed at which approved housing sites are developed. It finds that where competition for land is intense, house-builders must assume the highest possible sale prices to make winning bids for sites. Such bids are viable only because the release of land is restricted by the planning system, while the release of homes is managed on a site-by-site basis by builders to achieve the target sales rates underpinning earlier land bids. These factors have combined to encourage caution about the capacity of local housing markets to absorb new-build supply. Even if the planning system released substantially more land, it may take some time before house-builders responded by building out faster. While the research predates the recent collapse in speculative house-building, it is argued that these findings are likely still to be valid once the development of homes for owner-occupation recovers from the current recession.