Structural appraisal of a building proposed for retention
Authorities increasingly expect retention to be tested before demolition is proposed. Here is what the structural appraisal has to establish, how to compare the options fairly and how to present the conclusion.
A structural appraisal for retention has to answer one question in a form a planning officer can weigh: can this building carry the proposed use, and if not, what would it take. Everything else in the document supports that. Authorities increasingly expect the appraisal before they will accept demolition as the starting point, and a proposal that asserts the existing frame is unsuitable without opening up, testing and calculating will be asked to prove it.
Commission the survey before the massing study, not after the design team has committed to a clear site.
Why retention is now the default question
Embodied carbon has moved the burden of proof. National policy supports making effective use of land and existing buildings, and a growing number of adopted plans and supplementary documents now ask applicants to test retention and refurbishment before proposing substantial demolition. In London the requirement is sharper, with whole life carbon assessment and circular economy reporting expected on larger schemes.
Recent high-profile decisions on major redevelopment in central London have turned on the adequacy of the retention appraisal rather than on the design of the replacement. Those decisions are indicative of how inspectors and ministers have approached the issue rather than binding precedent, but the direction is clear enough to plan around.
The practical consequence is a document nobody was producing five years ago. An options appraisal that tests retention seriously, with structural evidence behind it, is now part of the cost of proposing demolition.
What the appraisal has to establish
Six findings, in this order.
- The structural form and materials as built, verified against archive drawings where they exist and by opening up where they do not.
- The current condition, including defects, movement, corrosion, water ingress and any previous alterations.
- The load capacity of floors, frame and foundations, calculated rather than assumed.
- The presence of deleterious or hazardous materials.
- The feasibility of the adaptations the scheme requires, including new openings, removed columns, added storeys and changed loading.
- The residual risk and the cost of resolving it.
As-built rarely matches as-designed. Beams get notched for services, columns get cut around, and mezzanines appear without calculation. The appraisal is a survey of what is there, and the number of opening-up locations is the variable that decides how much it is worth.
Testing depends on the material. Concrete frames need cover measurement, carbonation depth, core samples for strength and, where corrosion is suspected, half-cell potential mapping. Steel frames need section measurement, connection inspection and, in older buildings, coupon sampling and chemical analysis, since pre-war steels are not reliably weldable. Timber needs moisture content and decay inspection. Cast iron needs identification before anyone assumes it behaves like steel.
A structural survey sets this baseline out, and councils usually expect a suitably qualified author for it, with professional review strongly advised.
Load capacity for a changed use
Imposed floor loads differ by use category under the Eurocode loading standard, and the direction of the change matters more than the absolute figure.
| Change of use | Typical direction of imposed load | Usual consequence |
|---|---|---|
| Office to residential | Reduction | Floors usually have capacity; the constraint is elsewhere |
| Residential to office | Increase | Strengthening or load path changes likely |
| Retail or storage to residential | Reduction | Capacity rarely the issue |
| Anything to assembly, gym or plant | Substantial increase | Local strengthening or transfer structures |
A reduction in imposed load is not the end of the analysis. Residential conversion adds partitions, wet areas with screeds, acoustic floor build-ups and sometimes a new stair core, all of which redistribute load. Adding storeys changes the foundation case, and on a building from the 1960s the foundation records are usually the least reliable part of the archive.
Lateral stability is the quiet problem. Removing internal walls to open up a plan can remove the stability system in a building that never had a designed frame for it. Establish how the building resists wind before any wall is marked for removal.
Floor to floor heights and plan depth
Geometry defeats more retention schemes than structural capacity does.
Commercial buildings of the 1960s and 1970s often have generous floor to floor heights, which is helpful, because residential conversion needs clear headroom plus a raised floor or service zone plus an acoustic separating construction. Where the floor to floor is tight, the arithmetic runs out and the conversion produces units with unacceptable ceiling heights.
Plan depth is the other constraint. A deep-plan office floorplate converts badly to residential because habitable rooms need daylight and outlook, and the middle of the plate has neither. The fix is atria, light wells or a reduced footprint, all of which cut floor area and change the viability case.
Say this plainly in the appraisal when it is true. An honest conclusion that the frame is sound but the geometry defeats the proposed use is far more persuasive than a manufactured structural objection, and it points the scheme towards a different use rather than towards demolition.
Deleterious materials and what they cost you
Four categories come up repeatedly and each changes the programme.
Asbestos is the most common and the most predictable. A refurbishment and demolition survey under the Control of Asbestos Regulations 2012 is needed before any intrusive work, and it belongs in the pre-redevelopment audit alongside the wider material inventory. The relationship between asbestos survey and audit before demolition is worth settling early, because the two are often commissioned separately and duplicate each other.
Reinforced autoclaved aerated concrete planks need identification and condition assessment, and where present they usually govern the retention decision for the affected floors. High alumina cement in precast beams, calcium chloride added to concrete in older construction, and woodwool permanent formwork each carry their own investigation requirement.
The cost effect is rarely the removal itself. It is the uncertainty premium a contractor prices against an incomplete survey, and the only way to reduce it is to open up more before tender.
Comparing retention against demolition
Set the comparison out on consistent terms or it will be dismissed as advocacy.
- Capital cost, with the same design stage and the same level of contingency applied to both options
- Programme, including the time to investigate and de-risk the existing structure
- Risk, listed as specific items with a cost and a likelihood rather than as a single percentage
- Embodied carbon, from a whole life carbon assessment prepared on a recognised methodology
- Floor area achievable under each option, and the quality of that area
- Operational performance of the resulting building
- Tax treatment, which differs between new build and conversion and should be checked with an adviser rather than assumed
The contingency point is where most comparisons quietly fail. A retention option carrying a 25 per cent risk allowance against a new build option carrying 10 per cent is not a comparison, it is a conclusion. If the uncertainty is real, reduce it with more investigation rather than pricing it in and calling the matter closed.
Presenting the conclusion in the application
Structure the submission so an officer can follow the reasoning without a structural engineer beside them.
Start with the baseline: what the building is, how it was built, its condition and its capacity, with the investigation record in an appendix. Then the requirement: what the proposed use asks of the structure. Then the gap between them, expressed as specific interventions. Then the options, including full retention, partial retention of frame or facade, and replacement, each with cost, programme, carbon and area. Then the conclusion, with the reasons stated in one paragraph.
Where partial retention is proposed, the temporary condition becomes a planning matter as well as a construction one, and the sequencing assumptions usually end up secured through temporary works and construction stability conditions.
The honest uncertainty to acknowledge is that a retention appraisal at planning stage is based on partial information. Say what remains unknown, say how it will be resolved, and propose the condition that resolves it. An appraisal that claims certainty it cannot have is the one an officer will test hardest.
To see which supporting documents a redevelopment is likely to need, enter the postcode on PlanScope and work from the report list it produces.
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.
Frequently asked questions
Why do councils now ask for a retention appraisal?
Because embodied carbon has shifted the starting point. National policy supports effective use of existing buildings, and a growing number of adopted plans and supplementary documents ask applicants to test refurbishment before proposing substantial demolition. In London, whole life carbon and circular economy reporting is expected on larger schemes, which makes the appraisal part of the evidence base.
Does office to residential conversion usually have enough floor capacity?
Generally yes, because imposed floor loads for residential use are lower than for offices. The structural constraint is more often lateral stability after internal walls are removed, or the added weight of screeds, acoustic build-ups and new cores. Geometry, particularly floor to floor height and plan depth, defeats more conversions than load capacity does.
What testing does a concrete frame need?
Cover measurement, carbonation depth, core samples for compressive strength and, where corrosion is suspected, half-cell potential mapping. Add identification of any high alumina cement, calcium chloride additive or reinforced autoclaved aerated concrete planks. The number of opening-up locations is the main variable determining how useful the appraisal will be.
How should retention and demolition be compared?
On consistent terms: the same design stage, the same contingency approach, and risk itemised with cost and likelihood rather than absorbed into a single percentage. Include capital cost, programme, embodied carbon on a recognised methodology, achievable floor area and quality, operational performance and tax treatment. An inflated contingency on the retention option reads as a conclusion rather than an analysis.
What if the frame is sound but the building will not convert?
Say so directly. A conclusion that the structure is capable but the floor to floor height or plan depth defeats the proposed use is more persuasive than a manufactured structural objection, and it usually points the scheme towards a different use rather than towards demolition. Officers read a great many appraisals and recognise the difference.
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.