Independent Review of Build Out: Final Report (Letwin Review)

Summary

Commissioned at Budget 2017 and presented to Parliament in October 2018 (Cm 9720), the Independent Review of Build Out by the Rt Hon Sir Oliver Letwin MP was tasked, in its terms of reference, to "explain the significant gap between housing completions and the amount of land allocated or permissioned in areas of high housing demand, and make recommendations for closing it." It is the UK government's most authoritative investigation of why permissioned developable land in high-demand areas is built out slowly — the mainstream counterpart to the Georgist land-banking critique, reaching a partly different diagnosis.

The review focused on the largest sites, because "the largest sites are dominated by the major house builders... and it is in relation to these major firms that concern has been expressed in some quarters about 'land banking' and 'intentional delay'."

Findings

Build out is strikingly slow on large sites. Letwin reports that "the median build out period on the large sites I investigated was 15.5 years," equivalent to a median of just 6.5% of the site built out each year. Cross-checked against a Molior dataset for London supplied by the Mayor, he found the sample was, if anything, fast: "The median percentage annual build out rate for London sites of over 1,000 homes in the Molior data-set was 3.2%." Very large, fully permissioned sites in the highest-demand market in Britain thus release only a few percent of their homes per year.

The cause is the market absorption rate, not speculation on land price. Letwin's central conclusion: "the homogeneity of the types and tenures of the homes on offer on these sites, and the limits on the rate at which the market will absorb such homogenous products, are the fundamental drivers of the slow rate of build out." Builders release homes at the rate the local market will buy them without a fall in price; selling faster would require cutting prices, which Letwin judged would "create very serious problems not only for the major house builders but also, potentially, for prices and financing in the housing market." His prescription is therefore diversity of product and tenure on large sites (new planning rules for sites above a size threshold), so that complementary absorption rates across housing categories raise the overall build-out rate — not price controls and not, primarily, anti-land-banking enforcement.

Other constraints are secondary. Reviewing transport, remediation, utilities, logistics, capital, materials and labour, Letwin concluded none would bind if the absorption-rate constraint were removed — "with one exception... namely, the availability of skilled labour" (notably bricklayers).

Why It Matters

The review documents, with government authority, that a large stock of fully permissioned developable land in the highest-demand English markets is deliberately held in a state of slow build-out — the "significant gap" between permissions and completions. That supports the claim that valuable land is used well below its potential even where demand is most acute, and it corroborates Prosper Australia's land-banking argument that "developers strategically delay financially feasible developments" to protect staged-release revenue.

Crucially, Letwin refines the mechanism. He did not find that builders withhold land betting on future price appreciation (the crude land-banking thesis); he found they meter out completions to avoid depressing the price of a homogenous product. That is a form of strategic supply restriction driven by market power over local price — closer to the option-value and speculative-vacancy logic than to simple hoarding, and it sits alongside, not against, the zoning-constraint reading of urban underuse.

Limits

• Letwin explicitly rejects the simplest "land banking for capital gain" story; citing this review as proof of speculative withholding overstates it. Its finding is that build-out is metered to the absorption rate — a price-protection behaviour, not a bet on appreciation.
• Scope is England's large permissioned sites; it does not measure vacant dwellings or price the underuse of already-developed parcels (see Gyourko & Krimmel for the latter).
• It is a policy review, not a peer-reviewed causal study, though its build-out data are quantitative and cross-checked.

See Also

• Land underuse and speculative vacancy persist in high-demand cities — the outcome page this supports
• Prosper Australia, Speculative Vacancies 11 · Land Speculation
• Cunningham (2006), Seattle real options — the option-value mechanism behind slow release
• LVT dampens land speculation

Sources

1. Oliver Letwin (2018), Independent Review of Build Out: Final Report, Cm 9720, presented to Parliament by the Ministry of Housing, Communities & Local Government and HM Treasury, October 2018. GOV.UK landing page · PDF — used for the terms of reference, the 15.5-year / 6.5% and 3.2% build-out figures (paras 1.1–1.6), the absorption-rate/homogeneity diagnosis (paras 1.7–1.8), and the land-banking framing (para 1.2). Crown copyright, Open Government Licence v3.0. PDF fetched and text extracted 2026-07-13; quotes verbatim-verified against it.