The Government’s ambition to deliver 1.5 million homes is easy to support. We need more homes, the industry wants to build them and the planning system is being reshaped to make that possible.
But there is an uncomfortable gap between ambition and delivery.
England added 208,600 net homes in 2024/25, 6% fewer than the year before. The OBR’s November 2025 forecast suggests delivery will recover, but only pass 300,000 homes in one year before 2031. On that trajectory, the total over this Parliament is closer to 1.2 million than 1.5 million.
That is not simply a planning problem. It is a viability problem.
Development has always been a business of fine margins. A scheme can look compelling at first pass and become unworkable after a relatively small change to build cost, sales value, programme or finance.
The difference now is that several of those variables have moved against developers at the same time.
Between Q3 2021 and Q3 2025, build costs increased by 21.8%, while sales values grew by 13.1%. The House Builders Federation has estimated that the cost of delivering a hypothetical 90 sqm low-rise home is now £76,000 higher than in 2020. Material and labour account for £37,000 of that increase, with the remainder spread across the Future Homes Standard, building regulations, nutrient neutrality, Biodiversity Net Gain, taxation, Section 106 inflation and the Building Safety Levy.
None of those requirements exists in isolation. Each may be understandable on its own. But a development appraisal has to absorb all of them at once.
That is why almost two-thirds of respondents to HBF research identified viability as a major barrier to delivery, and why Zoopla’s modelling found viability challenges across almost two-thirds of English local authority areas.
This is no longer an issue confined to a few difficult markets or unusual sites. It is a national delivery constraint.
The draft NPPF seeks to reduce unnecessary site-specific viability assessments. The intention is sensible: policies should be tested at plan-making stage, land should be bought with those policies in mind and viability should not become a routine way to renegotiate obligations.
The difficulty is that a plan-level viability assessment is a snapshot.
Lichfields’ review of 144 Local Plan and CIL viability studies found that 65% were more than five years old, with a median age of 5.83 years. Five years is a long time in development. Interest rates can change. Build costs can move. Sales markets can stall. New regulatory costs can be introduced. A site can reveal constraints that were not visible when the plan was adopted.
The assumption that a policy-compliant proposal is viable is only as reliable as the assumptions beneath it.
That does not mean every scheme should reopen every obligation. It means we need to distinguish between opportunistic viability arguments and genuine changes in the economics of delivery. If circumstances have moved materially, revisiting viability is not an attempt to avoid policy. It is a necessary test of whether the scheme can actually happen.
Too often, viability is treated as a document produced near the end of a process. In reality, the most useful viability work happens much earlier.
It starts when a site is being assessed and the team is still able to change the land offer, design, tenure mix, specification, programme or funding structure. It continues as better information becomes available. And it should give everyone involved a clear view of which assumptions have changed and what those changes do to the result.
This matters because a single headline return tells you very little on its own. You need to see the sensitivity: what happens if build costs rise by another 5%? What if sales are delayed by six months? What if the affordable housing mix changes? What if finance costs remain higher for longer?
The point is not to model every imaginable downside. It is to understand which assumptions carry the scheme and how much room there is for them to move.
That requires three things: consistency, transparency and speed.
Consistency means the same calculation logic is applied across sites and teams. Transparency means assumptions and sources can be seen, challenged and explained. Speed means the appraisal can be updated while a decision is still live, rather than after the opportunity has passed.
The draft NPPF’s emphasis on standardised inputs has value. A shared framework makes it easier for developers, valuers and local authorities to understand where views genuinely differ. It reduces time lost debating the structure of the model and puts the focus on the evidence.
But standardisation should not be confused with pretending every site is standard.
A typology can provide a baseline. It cannot capture every abnormal cost, ground condition, phasing challenge or market shift. The professional task is to start from a consistent framework, then clearly justify where and why the reality of a site departs from it.
That is also where better technology has a practical role. Not in deciding whether a site should proceed, and not in replacing professional judgment, but in giving that judgment a clearer and more reliable foundation. When the model is structured, auditable and easy to update, teams spend less time checking formulas and reconciling versions, and more time examining the assumptions that actually determine whether homes get built.
Housing targets matter. Planning reform matters. Policy requirements matter too.
But none of them delivers a home unless a scheme can move from an allocation, to a consent, to a funded construction project.
My view is that viability needs to be treated as a continuous discipline, not a one-off hurdle. Test it early. Revisit it when the facts change. Make the assumptions visible. Keep the methodology consistent. And use sensitivity analysis to understand the margin before the market finds it for you.
Because ambition and viability are not competing ideas. Viability is the route by which ambition becomes delivery.