Optimum viable use, conservation deficit and enabling development

Optimum viable use for a listed building is the use that keeps the building standing with the least harm to what makes it special, not the use that makes the most money. That distinction decides most difficult heritage cases. Where a building is decaying and no conventional scheme stacks up, the council is being asked to accept harm now in exchange for conservation later, and the policy framework gives it a structured way to test whether that bargain is real.

The test is harder than owners expect, and it is deliberately hostile to one particular argument: that the purchase price makes the numbers impossible.

What optimum viable use means in policy

The historic environment chapter of the National Planning Policy Framework, in its August 2026 edition, asks decision-takers to weigh harm to a designated heritage asset against public benefits, and treats the asset's viable use as part of its conservation. The Planning Practice Guidance on the historic environment develops the point: where a building has no viable use, decay follows, so finding a use that is both viable and sympathetic is itself a conservation outcome.

Optimum viable use is the use that causes the least harm to significance among those that are actually viable. Three things follow.

It is not necessarily the original use. A chapel will rarely return to worship, and insisting on it can guarantee decay.

It is not the most profitable use either. Where two uses are both viable, the one causing less harm to significance is the optimum, even if it returns less.

And viability here means a realistic prospect of a long-term use with an owner able to maintain the building, judged on evidence rather than on an owner's preference.

Under section 16 of the Planning (Listed Buildings and Conservation Areas) Act 1990, the decision-taker must have special regard to the desirability of preserving the building, its setting and any features of special interest. That duty does not disappear because a viability case has been made.

The conservation deficit and how it is evidenced

A conservation deficit exists where the cost of putting a building into sustainable use exceeds what it will be worth afterwards. The arithmetic is simple and the inputs are where the argument lives.

Subtract the end value from the sum of the costs and the return. A positive figure is the deficit, and it is the maximum that enabling development can legitimately be asked to fund.

The point that defeats most applications is the existing value line. Policy looks at the inherent needs of the building, so a deficit created by paying a hope-value price is not a deficit the planning system has to cure. An owner who bought at residential land value and now says repairs cannot be afforded is describing a bad bargain, not a conservation problem.

A Viability Assessment prepared for this purpose needs to be legible to a reviewer appointed by the council, which in practice means following the RICS professional statement on financial viability in planning and expecting the appraisal to be published.

Enabling development and the tests applied

Enabling development is development that would otherwise be refused, permitted because it funds the conservation of a heritage asset. Historic England's advice sets out criteria that councils and inspectors apply closely, and the structure of a successful case follows them in order.

1. The enabling development will not materially harm the heritage values of the place or its setting.
2. It avoids fragmenting the management of the place between different owners.
3. It secures the long-term future of the place, and where relevant its continued use for a sympathetic purpose.
4. It is necessary because of the inherent needs of the place, not the circumstances of the present owner or the price paid.
5. No sufficient subsidy is available from any other source, including grant funding.
6. The amount of enabling development is the minimum necessary, in a form that minimises harm to other public interests.
7. The public benefit of securing the future of the place decisively outweighs the harm caused by departing from other policies.

Criterion six is where schemes are trimmed. If the deficit is £900,000 and the enabling element would generate £1.6 million, the surplus is not a reason to approve a larger scheme. Expect the council to cut the unit count, and build the appraisal so that it still works after the cut.

The wider mechanics of funding repair with new build, including where the new element should sit on the site, are covered in our note on enabling development.

Securing the heritage benefit

A permission that allows the new houses to be built and sold before the historic building is repaired is worthless as conservation, and councils know it. The planning obligation and the condition schedule carry the whole burden.

What works in practice is a sequence tied to physical milestones rather than to dates. No occupation of the enabling development until the repair works to the listed building have been completed and certified by the council's conservation officer. Where phasing is unavoidable, split the enabling element so that each tranche is released on completion of a defined repair package.

Add a bond or a retention where the applicant's covenant is weak. Add a management and maintenance plan with a term long enough to matter. And resist the temptation to accept a financial contribution instead of works, because a contribution paid to the council does not repair a roof.

Historic Building Recording to the appropriate Historic England level is frequently conditioned alongside, since significant fabric will be lost or concealed during the works and the record is the mitigation.

Viability evidence and its scrutiny

Expect the council to appoint its own reviewer and to charge you for it. Expect the appraisal to be published, subject only to narrow commercial redactions. The Planning Practice Guidance on viability treats transparency as the default, and a refusal to publish reads as a concession that the numbers do not bear reading.

The arguments that reviewers test hardest are the build cost per square metre against published tender data, the developer's return against the actual risk profile, the end value against local comparables rather than aspiration, and the treatment of value attributable to the heritage asset itself. A restored Georgian townhouse carries a premium, and an appraisal that ignores it overstates the deficit.

One honest uncertainty: councils vary widely in how they treat an owner's own purchase price when it sits above existing use value. Some discount it entirely, which is the orthodox reading of the policy. Others take a more pragmatic view where the building is on the Heritage at Risk Register and no other purchaser exists. Ask at pre-application rather than guessing.

Where the argument fails

Deliberate neglect is the clearest failure. National policy is explicit that where there is evidence of deliberate neglect of, or damage to, a heritage asset, the deteriorated state of the asset should not be taken into account in the decision. An owner who has watched a roof fail for a decade cannot then rely on the consequences.

Councils also have tools that change the negotiation. An urgent works notice under section 54 of the 1990 Act allows the authority to carry out works to preserve an unoccupied building and recover the cost. A repairs notice can precede compulsory purchase. Neither is used often, but both are available where an owner's case is that nothing can be done.

The other common failure is scope creep. A scheme that starts as repair of a barn group and ends as fourteen houses in an open field has stopped being enabling development and become a housing application with a heritage justification attached. Inspectors see the difference.

Conditions, phasing and the paper trail

Draft the condition schedule yourself and submit it. The documents the council will want are a statement of significance, a schedule of repairs with costings, the viability appraisal, a structural report where collapse is in issue, and a phasing plan linking the two parts of the scheme.

Illustrative example: a Grade II farmhouse and threshing barn on the Heritage at Risk Register, with a priced repair schedule of £1.1 million against a restored value of £750,000. The deficit of around £350,000 before return supports a small enabling element of three dwellings on the former yard, conditioned so that none may be occupied until the farmhouse roof, walls and windows are signed off. A fourth dwelling, offered as contingency, is refused as exceeding the minimum necessary.

A Heritage Statement drafted on PlanScope will set out significance, the harm analysis and the policy tests, and will flag the site facts it cannot supply. Professional review is strongly advised before submission on any asset where the viability case carries the application.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.