Structural survey before buying a site with a retained building

A structural survey before site purchase has one job, and it is not to price the repairs. On an acquisition where a building is being retained, the survey has to tell you whether the scheme in your appraisal is physically possible on that frame, those floors and those foundations, and what you cannot find out before exchange. Those two answers decide whether you buy, at what price, and on what conditions.

Most of the money is lost on the second one. A survey that reads as reassuring because nobody could open anything up is not a reassuring survey.

What to commission before exchange

A Structural Survey scoped to the proposal is the starting point, not a condition report. Brief the engineer with the scheme: two added storeys, a change of use to residential, a new core through the slab, whatever it is. An appraisal written against the proposal tells you about capacity. One written against the existing building tells you about defects, which is a different and much less useful document.

Alongside it, five other pieces of work usually earn their fee on a retained building.

• A document trawl for original drawings, calculations, as-built records and any previous building control or warranty paperwork. The local authority building control file and the Land Registry title are both worth searching.
• An asbestos management survey, moving to a refurbishment and demolition survey before works, because the Control of Asbestos Regulations 2012 duty sits with the duty holder from the day you own it.
• A deleterious materials review. Reinforced autoclaved aerated concrete planks in roofs and floors of buildings from the 1950s to the 1980s, high alumina cement, woodwool shuttering and calcium silicate brickwork all change a scheme, and RAAC in particular can make retention uneconomic on its own.
• Ground investigation where loads will increase, reported to Eurocode 7, because added storeys on pad foundations sized for the original building is the most common fatal finding. A Geotechnical Assessment costs very little relative to discovering the problem after exchange.
• Drainage, by CCTV survey, because a drain run under the retained footprint that has to be diverted is a programme item and a cost.

Councils normally expect a suitably qualified structural engineer to author anything that goes into a planning application, and professional review is strongly advised on any draft.

Access and opening up during a marketing period

This is the constraint that shapes everything. A vendor marketing a tenanted or occupied building will allow a walkover, sometimes a measured survey, and almost never the removal of finishes, the drilling of cores or the digging of trial pits.

So the engineer works from visual inspection, documents and judgement, and the report carries limitations that are genuine rather than defensive. Typical ones include no inspection of foundations, no verification of reinforcement or slab thickness, concealed structure not inspected, no confirmation of lateral stability connections, and floor loading assumed from the apparent construction era rather than measured.

Four ways to manage that gap, in descending order of preference:

1. Negotiate an access licence during exclusivity that permits specified intrusive works, with reinstatement and an indemnity. Vendors agree to this more often than buyers ask.
2. Make the contract conditional on satisfactory intrusive investigation, with a defined scope and a defined period.
3. Agree a price retention released on the results, with a mechanism for resolving disagreement.
4. Price the risk and buy with it, which is only rational where the downside is bounded and quantified.

What does not work is treating the limitations section as boilerplate. Read it, list every item the engineer could not verify, and put a figure and a consequence against each one.

Reading a survey for deal risk rather than repair cost

Separate the findings into three buckets before you look at any numbers.

Deal breakers are items that make the appraised scheme impossible or uneconomic. Frame capacity insufficient for the added storeys without a new structure. Floor-to-floor heights too tight for residential once services and acoustic separation are allowed for. Foundations that cannot take the load without underpinning or new piles. RAAC in the retained roof. Lateral stability provided by elements the scheme proposes to remove.

Deal shapers change the scheme but not the decision. A core that has to move, a transfer structure, strengthening to specific bays, a facade retained rather than reclad.

Repairs are just cost, and in a development appraisal they are usually the smallest of the three.

Then ask the engineer one direct question: what would change your opinion most if we could open up one thing? The answer tells you what to negotiate access for.

Items that change the development appraisal

Five findings move numbers enough to warrant a re-run of the appraisal rather than a line item.

Added storeys needing new foundations rather than frame strengthening. The cost difference is an order of magnitude, and it usually brings a basement or piling exercise beside existing structures with it.

Floor-to-ceiling height below what a residential conversion needs once a raised floor and a suspended ceiling are allowed for. This kills more office to residential schemes than planning policy does.

Cladding replacement on a taller building, which brings the building safety regime into the programme. Where the scheme involves a relevant building, a Fire Statement is required at Planning Gateway One under the Building Safety Act 2022 framework and Article 9A of the Development Management Procedure Order 2015. That document has to be prepared by someone with the necessary fire engineering competence, and it is legally required rather than advisable, so budget for the fire engineer from the outset.

Drainage or service diversions under the retained footprint, which frequently need a build over or build near agreement. The checks are the same as for building over or near buried infrastructure on any site, and the agreements take months rather than weeks.

Asbestos in a form that requires licensed removal before any structural work, which puts a fixed sequence and a fixed duration at the front of the programme.

Conditions and warranties to negotiate

Three things are worth pushing for on a retained-building acquisition.

Assignment or collateral warranties covering the existing structure, where any exist. On a building with recent structural works there may be a designer's appointment, a contractor's warranty or an insurance-backed product, and those are assets with value. Keeping the paperwork together matters afterwards too, and there is a reason structural warranties belong in the planning file rather than in a solicitor's archive.

Vendor warranties on specific known matters, narrowly drawn. A general warranty that the building is in good condition is worth little. A warranty that no RAAC is present, or that no structural alterations were carried out without building control approval since a stated date, is worth something because it is capable of being breached identifiably.

Retentions tied to the results of defined investigations, with the scope and the release mechanism agreed before exchange rather than described in principle.

Ask separately for the asbestos register, the fire risk assessment, any structural reports commissioned by the vendor and the building control completion certificates. And a vendor who will not disclose a report they have commissioned has told you something already.

Sequencing with the planning strategy

The order matters more than the budget. Structural capacity sets the envelope, the envelope sets the planning strategy, and the planning strategy sets the application documents. Running them in the other direction produces a consented scheme that cannot be built on the structure you own.

A workable sequence on a retained building is to establish the structural envelope first, test it against the planning constraints in a short feasibility, decide between a full application, a prior approval route and a hybrid, and only then commission the supporting documents. Where a change of use under a prior approval route is in play, note that the prior approval tests are narrower than a full application and will not accommodate a scheme the structure cannot carry.

One honest uncertainty. However good the pre-exchange work, something will be found when the finishes come off, and on a retained building of any age the sensible contingency is larger than the one your appraisal template suggests.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Working out which documents a retained-building scheme needs? Enter your postcode on PlanScope to see what the site is likely to require.

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