Retain or demolish: the evidence authorities now expect
Authorities increasingly test whether an existing building could be retained before accepting demolition. The structural investigation, the height and floorplate limits, and the options appraisal that persuades.
The retain or demolish existing structure question now has to be answered with evidence rather than with a preference. On larger schemes, and almost always in London, an authority will ask what was considered before the applicant settled on demolition and rebuild, and a design and access statement that simply asserts the existing building is unsuitable is a weak answer. The useful version sets out what the frame can carry, what floor to floor heights allow, what the carbon arithmetic says, and why retention was ruled out.
That evidence is structural first and policy second. An authority cannot be persuaded by a carbon number alone if the building physically cannot take the loads, and it will not accept a structural objection that turns out to rest on an assumption nobody tested.
Why authorities now ask the retention question
Three pressures converge. Embodied carbon in a new frame is large and immediate, while operational savings accrue slowly, so whole life carbon accounting tends to favour reuse. Waste policy pushes the same way. And local plans increasingly carry a policy expecting the reuse of existing buildings to be considered before redevelopment.
In London the framework is explicit. London Plan policy SI 2 requires whole life carbon assessments for referable development, and policy SI 7 deals with waste reduction and the circular economy, which is where the retention option gets tested. Outside London the position varies by local plan, so read the adopted development plan rather than assuming.
The practical effect is that the demolition case has become a piece of submitted evidence in its own right. A Pre-redevelopment Audit recording what is on site and what could be retained or reclaimed is often the document that makes or breaks the argument.
What an existing frame can actually carry
Start with the structure as built, not as drawn. Archive drawings tell you the design intent, and sixty year old buildings frequently differ from them.
The investigation that supports a retention decision usually involves:
- Reviewing archive drawings, calculations and any previous alteration records.
- Opening up at sample locations to confirm element sizes, slab thicknesses and connection details.
- Scanning for reinforcement and measuring cover, with core samples for concrete strength where the frame is in question.
- Testing for high alumina cement or other period specific materials where the construction date suggests the risk.
- Assessing the existing foundations, often the binding constraint, including pile records if any survive.
- Checking lateral stability, since many older frames rely on cladding or partitions that the new scheme will remove.
Foundations are where retention schemes most often fail. An existing frame may take a modest increase in floor loading, but adding storeys usually asks more of the foundations than they were designed for, and underpinning or supplementary piling inside an occupied footprint is expensive and slow. Where piling is proposed near retained structure, expect the authority to condition the method, and the structural survey needs to anticipate that.
Assessment of an existing structure is not the same exercise as design of a new one. Partial factors, material testing and the treatment of as built dimensions follow the Eurocode framework for assessment of existing structures, and the appointed engineer should state which assessment route was used.
Floor to floor heights and what you can build inside them
Height between floors sets the usable outcome more often than load capacity does. An office frame with 3.0 metre floor to floor leaves little room for a residential build-up once you allow for a raised or acoustically isolated floor, services, insulation and a sensible ceiling zone.
The constraints to test are floor to ceiling height against local policy and the Nationally Described Space Standard where it applies, separating floor construction for sound insulation between dwellings, horizontal service distribution without dropping ceilings in habitable rooms, and window head heights relative to the proposed internal floor level. Deep plan floorplates add another problem, because residential layouts need daylight to habitable rooms and an office floorplate 20 metres deep cannot provide it without cutting a courtyard or an atrium.
Those cuts are not necessarily bad. Removing a bay or opening a light well can turn an unusable floorplate into a workable one, and the carbon cost of local demolition is far below that of a whole new frame.
| Existing building trait | Retention prospect |
|---|---|
| Generous floor to floor, regular grid | Strong |
| Sound foundations, low existing loading | Strong |
| Deep plan with no light wells | Workable with surgery |
| Floor to floor under about 2.9 metres for residential | Difficult |
| Foundations at capacity with added storeys proposed | Difficult |
| Confirmed structural defects in the primary frame | Weak |
Treat that as a starting pattern. Every one of those judgements depends on survey evidence rather than on the building's age or appearance.
Building the demolition case
If demolition is the right answer, prove it in the order an officer will read it.
Set out the existing building and its condition, supported by survey. Set out the brief the site has to deliver, by use, quantum and policy compliance. Then show the retention options you tested, with enough design to be credible: full retention, partial retention of frame or facade, retention with extension, and complete redevelopment. Give each one a whole life carbon figure, an area schedule and a note of why it fails or succeeds against the brief.
The common error is to test one retention option, badly, and then dismiss the category. Officers see that regularly and discount it. The other common error is to rely on viability alone. Cost is a material consideration but a retention option that is merely less profitable is not the same as one that is unachievable, and that distinction gets tested at committee.
A whole life carbon comparison is the quantitative spine of the argument. Prepared to the RICS methodology, it covers the product and construction stages, the use stage and end of life, which is the only way the embodied cost of a new frame can be weighed against the operational performance it delivers. PlanScope can draft a Whole Life Carbon Assessment structure for that comparison, and a Circular Economy Statement where the local requirement calls for one.
Where retention is not available
Some buildings cannot be kept, and saying so plainly is better than a hedged paragraph. Confirmed structural failure, extensive concrete deterioration, a frame built with materials that cannot be left in a residential building, contamination within the structure, or a footprint that cannot deliver the policy compliant mix all point one way.
Hazardous materials deserve their own evidence. A refurbishment and demolition asbestos survey is needed before works begin in any event, and the findings change both the programme and the retention case. There is a separate note on demolition asbestos surveys and the planning submission that sets out what the authority sees and when.
Be careful with the consents. Demolition of an unlisted building outside a conservation area may be prior approval development under the General Permitted Development Order 2015, while relevant demolition in a conservation area needs planning permission in its own right, and anything affecting a listed building needs listed building consent. Tax treatment also differs between new build and refurbishment, so take advice before the structural decision is locked, because it can move the arithmetic.
What to submit
The pack that answers the retention question usually contains a structural appraisal of the existing building, a pre-redevelopment audit of materials and components, a whole life carbon assessment comparing the options, a design statement explaining the floorplate and height constraints, and a short options appraisal that an officer can follow without opening the appendices.
Put the options appraisal at the front. Officers and members read the first three pages properly, and a one page table comparing retention scenarios against carbon, area and deliverability does more work than forty pages of calculation behind it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required: the structural assessment behind a retention decision is engineering work, and the draft is a starting point for that engineer rather than a substitute.
Frequently asked questions
Is there a national policy requiring retention before demolition?
There is no single national rule that bars demolition. National waste and carbon policy, and a growing number of adopted local plans, expect the reuse option to be considered and justified. In London, London Plan policies SI 2 and SI 7 bring whole life carbon and circular economy testing into the decision, so read the adopted development plan for your authority.
What usually prevents an existing frame being reused?
Foundations are the most frequent constraint, because adding storeys asks more of them than they were designed for. Low floor to floor heights, deep floorplates with no daylight to the centre, lateral stability that relied on removed partitions, and confirmed deterioration of the primary frame are the other common blockers.
How much investigation is needed before the planning application?
Enough to support the conclusion. That normally means archive drawing review, opening up at sample locations, reinforcement scanning and cover measurement, concrete testing where strength is in doubt, a foundation assessment, and a check for period specific materials. An assertion without sample investigation is routinely challenged.
Does a weak viability case justify demolition?
Not on its own. Cost is a material consideration, but an option that is less profitable differs from one that cannot be delivered, and that distinction gets tested at committee. Pair any viability argument with structural and carbon evidence rather than relying on it alone.
Do I need permission to demolish?
It depends on the building and the area. Demolition of some unlisted buildings outside conservation areas is prior approval development under the General Permitted Development Order 2015. Relevant demolition in a conservation area needs planning permission, and works to a listed building need listed building consent.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.