Retaining existing structure: what an appraisal must establish

An existing structure retention appraisal has one job at planning stage: to establish, with evidence, whether the frame can carry the proposed scheme, and at what cost and programme penalty compared with demolition. That is a narrower question than "is the building sound", and it is the question a planning officer asking about retrofit will actually put to you.

Get the scope wrong and you end up with a document that says the structure is in reasonable condition, which answers nothing, and a planning committee that reads the absence of a conclusion as a reason to refuse the demolition.

Why retention is being tested

Two pressures have converged. Local plan policy increasingly asks applicants to demonstrate that reuse has been considered before demolition is accepted, and in London the referable-scheme regime asks for a comparison of retention against demolition supported by carbon figures. The carbon case runs through a Whole Life Carbon Assessment, which needs the structural conclusion as an input rather than the other way round.

The practical consequence for a developer is sequencing. The structural appraisal has to be far enough advanced to give the carbon assessment a real option to compare, and both have to be done before the design is fixed, because a scheme drawn on a demolished site cannot be retro-fitted onto a retained frame.

Demolition itself is not always free of consent either. Relevant demolition in a conservation area needs planning permission, and demolition elsewhere may need prior approval under Part 11 of Schedule 2 to the General Permitted Development Order 2015 for the method and restoration of the site.

Record drawings and how far to trust them

Start with the paper, then stop trusting it.

Record drawings for a building put up between the 1950s and the 1980s are usually available from the local authority building control archive, the original engineer's successor practice, or the tenant's own facilities file. They tell you the design intent: grid, member sizes, slab depths, the design code in force, and often the assumed imposed loads.

What they do not tell you is what was built. Substitutions during construction, later alterations, openings cut through slabs for services, removed columns, and strengthening carried out for a previous tenant are all common and rarely documented. On a 1960s reinforced concrete frame the reinforcement detail matters more than the member size, and reinforcement drawings survive less often than general arrangements.

So treat the records as a hypothesis to test. The appraisal should state, drawing by drawing, whether the record has been verified on site, partially verified, or not verified at all. An appraisal that presents unverified record information as fact is the single most common defect in these documents.

Intrusive investigation and what it buys

Non-intrusive work comes first: dimensional survey, usually by laser scan, cover meter or ground-penetrating radar to locate reinforcement, hammer testing, and a visual condition survey of exposed elements. That establishes geometry and flags the areas worth opening up.

Intrusive investigation then answers the questions that matter:

• Concrete strength from cores tested to the relevant standard, plus carbonation depth and chloride content where corrosion is suspected.
• Reinforcement size, grade, spacing and cover, confirmed by breaking out at representative locations, including at least one critical location such as a column head or a beam support.
• Steel section identification, connection detail and, where the design has to be verified, material properties from coupon testing.
• Condition of concealed elements: bearings, movement joints, embedded steelwork, and the slab soffit above suspended ceilings.

Access is the cost driver, not the testing. On an occupied or part-occupied building the appraisal has to be programmed around tenant access, out-of-hours working and making good, and that is usually a larger line than the laboratory fees. Budget the enabling works and the reinstatement separately from the engineering fee so that the number is not a surprise.

Where the building sits over a basement or where a new basement is proposed, the structural work overlaps with ground investigation, and doing them as one campaign saves a second mobilisation.

Capacity against the proposed loading

This is the core of the appraisal and the part most often skipped at planning stage.

The original frame was designed to a superseded code with the imposed loads of its time. An office floor designed in 1968 was designed for the office use of 1968, and a residential conversion, a gym, a plant deck or a green roof changes the demand. Assessment against current Eurocode requirements, using verified material properties and the actual geometry, produces one of three answers for each element: adequate, adequate with local strengthening, or inadequate.

Report it element by element and area by area, not as a single verdict. A frame where 80 per cent of the floor plate is adequate and the transfer structure at the entrance is not is a very different proposition from one where the columns are undersized throughout. State the assumed loading you tested against, and state it in the same terms the architect is using, because a mismatch between the structural assumption and the drawn use is how these documents come apart under scrutiny.

Two items deserve their own treatment. Lateral stability, because retained frames often relied on cladding or infill panels that the new scheme removes. And disproportionate collapse requirements, which apply to the altered building and can be the reason a retention scheme needs new tying or a new core.

Tolerances, alignment and buildability

The appraisal has to say whether the retained structure can be built against, as well as whether it stands up.

Existing frames are out of plumb and out of level by amounts that modern construction does not assume. A floor with a 40 millimetre dish across a bay is structurally unremarkable and a serious problem for a bathroom pod, a lift shaft or a unitised facade. Grid lines that were set out to imperial dimensions do not accept a metric cladding module without a packing strategy.

Set out in the appraisal what has been measured, to what accuracy, and what allowance the design needs to carry. Then say who owns the risk of what has not been measured, because on a retention job that allocation is the commercial heart of the contract.

The related evidence question for an upward extension is covered in our notes on airspace development above flats and on roof extensions and structural capacity evidence.

Where retention is possible but abortive

Sometimes the honest conclusion is that the frame can be retained and that doing so produces a worse building.

The reasons are usually floor-to-floor height, plan efficiency, or a column grid that will not take a residential layout without generating unlettable space. A 2.6 metre floor-to-floor office frame cannot accommodate a residential ceiling height plus a services zone plus a screed. No amount of strengthening changes that.

Say so plainly, with the dimensional evidence, and quantify the loss. A planning authority will accept a reasoned case that retention has been tested and fails on a measurable ground far more readily than it will accept a general assertion that the building is at the end of its life. And a carbon comparison that includes the operational penalty of a compromised retained building is a fairer comparison than one that counts only embodied carbon.

Reporting the conclusion for a planning audience

The reader is a planning officer or a committee member, not an engineer. Structure the document accordingly.

1. A short statement of what was asked and what scheme was tested.
2. What is known, what was verified and how, with the limitations listed rather than buried.
3. The element by element capacity conclusion in a table a non-engineer can read.
4. The retention options considered, each with its structural implications, and the reason any was discounted.
5. The recommended option and the residual risks, with the further investigation needed at the next stage named.

Where a planning application relies on this work, the submitted Structural Survey or structural appraisal needs to be signed by a suitably qualified engineer. A chartered structural engineer with appraisal experience is the expected author, and councils routinely give little weight to an unsigned or unattributed report.

One honest uncertainty. Appraisal never removes risk entirely, because you cannot open up everything, and there is always a chance the worst element is the one not tested. The appraisal's job is to size that residual risk and say where it sits, not to pretend it is zero.

This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required, and a structural appraisal should be prepared and signed by a suitably qualified engineer.

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