Structural survey against geotechnical assessment
Conversion schemes routinely confuse the two. Here is what each report concludes, which situations make planning ask for one, and how they overlap on movement and cracking.
A structural survey looks at the building. A geotechnical assessment looks at what the building is standing on. On a conversion scheme those two questions get merged into one line item in a fee proposal and then one of them quietly does not happen, usually the ground one, and the gap shows up when the foundations are dug.
For planning purposes the distinction matters because they answer different objections. A structural appraisal answers "can this building be converted without effectively rebuilding it". A geotechnical report answers "can this site take what you are proposing to put on it". Councils ask for the building question far more often than the ground one.
Building fabric against ground conditions
A structural appraisal is carried out by a structural engineer inspecting the existing structure. They look at the load paths, the condition of walls, frames, floors and roof, the nature and cause of any cracking, timber decay, corrosion of embedded steel, the adequacy of lateral restraint, and whether the fabric can carry the loads a conversion would impose. Opening up is sometimes needed. Crack monitoring over several months is sometimes needed.
A geotechnical assessment is carried out by a geotechnical engineer using ground investigation data. Trial pits and boreholes establish the soil profile, strength, plasticity and groundwater level, with laboratory testing behind it. The report then interprets that data for foundation design under Eurocode 7, comments on slope stability, shrinkable clay and any mining or dissolution risk, and where drainage relies on infiltration it reports soakaway testing.
The two disciplines overlap in the people who practise them and diverge entirely in the evidence they rest on. One is a building inspection. The other is a set of holes and a laboratory schedule.
What each report concludes
| Question | Structural appraisal | Geotechnical assessment |
|---|---|---|
| Primary subject | The existing building | The ground beneath and around it |
| Typical method | Visual inspection, opening up, monitoring | Trial pits, boreholes, laboratory testing |
| Governing reference | Engineering appraisal practice for existing structures | BS 5930 for investigation, Eurocode 7 for design |
| Planning use | Feasibility of conversion or retention | Foundation feasibility, stability, drainage |
| Common trigger | Barn or building conversion, demolition claims | Basements, sloping sites, mining areas, soakaways |
A structural survey concludes with a view on whether the building can be converted, what strengthening is needed, and whether any element must be rebuilt. A geotechnical assessment concludes with recommended foundation types and depths, bearing values, and any constraints on the layout arising from stability or ground water.
Which one planning actually needs
Most householder and minor applications need neither. Structural adequacy and foundation design are Building Regulations matters, assessed at building control rather than by a case officer.
Four situations change that.
Conversions under the agricultural to residential permitted development route are the common one. The right does not permit works that amount to rebuilding, and prior approval turns on whether the existing building is capable of conversion. A structural appraisal that states clearly which elements are being retained, and confirms the frame and walls can take the loading the conversion brings, is the evidence that decides the application. Appeals on this point turn on exactly that question.
Demolition in a conservation area, or of a non-designated heritage asset, is the second. Where an applicant argues a building is beyond repair, the council will want an engineer's assessment with repair costings rather than an assertion.
Basements are the third. Many authorities, particularly in London, require a basement impact assessment covering ground conditions, groundwater flow, stability of neighbouring structures and surface water, which is a geotechnical document with a structural component. A Basement Impact Assessment is a distinct report from either of the two discussed here and usually needs both inputs.
Land stability is the fourth. Sites on steep slopes, in coalfield referral areas, on dissolution features or near quarries and landfill attract a ground stability requirement in national guidance, and the council will expect the risk to be addressed at application stage rather than conditioned away.
Overlap on subsidence and movement
Cracking is where the two reports need each other. A structural engineer can describe a crack pattern and infer a mechanism, but distinguishing clay shrinkage from a leaking drain, from a nearby tree, from historic mining or from thermal movement usually needs ground data.
On a site with mature trees on shrinkable clay, the sequence that works is: structural inspection first to characterise the movement, then targeted trial pits with root identification and moisture content testing, then a foundation design that responds to both. Running the ground investigation without the structural inspection produces a report that answers a question nobody asked.
Where contamination is also suspected, combine the visits. The same rig that takes geotechnical samples can take samples for chemical analysis, and what triggers the intrusive stage is worth settling before you mobilise, because a second mobilisation costs more than the extra samples.
Costs and site work involved
A structural appraisal of a single building is a site visit and a report. Costs rise where opening up is needed, where access equipment is required to inspect a roof, or where monitoring runs over a season. The programme is days to weeks, unless monitoring is involved, in which case it is months.
A geotechnical assessment carries plant costs. You are paying for a rig or an excavator, an engineer on site, utility clearance before any hole is dug, sometimes traffic management, laboratory analysis and the interpretive reporting afterwards. It is the more expensive of the two in almost every case, and it needs site access, which on an occupied site or a tenanted farm takes arranging.
Both are reports councils expect from suitably qualified authors, and professional review is strongly advised. An AI-assisted draft can structure the report, assemble the policy and standards context and mark every place a measured value is missing, and a chartered engineer then has to carry out the inspection or the interpretation and sign it.
How each feeds the design
The structural appraisal shapes the drawings. It tells you which openings are possible, whether the roof can carry a new covering, whether a wall can be removed without a transfer structure, and how much of the existing fabric can actually be retained. On a conversion that answer changes the floor plan, and it is better known before the planning drawings are frozen than after.
The geotechnical assessment shapes the sections and the drainage strategy. Foundation depth on shrinkable clay near trees affects levels and thresholds. Groundwater near the surface rules out an infiltration-based drainage strategy and pushes you towards attenuation and a discharge agreement. Slope stability affects where you can cut and fill.
Sequence them early. Both reports cost less to act on at sketch stage and are painful to act on once a permission has fixed the layout. A week of engineering time in month one routinely saves a section 73 application in month fourteen.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. If you want to see which technical reports a conversion is likely to trigger, enter your postcode on PlanScope.
Frequently asked questions
Does a planning application need a structural survey?
Most do not, because structural adequacy is a Building Regulations matter assessed at building control. It becomes necessary for agricultural building conversions under permitted development, where an applicant argues a building is beyond repair, and where a council needs evidence that a retained structure can carry the proposed loads.
What is the difference between the two reports?
A structural appraisal inspects the existing building: load paths, wall and frame condition, cracking, decay and capacity for new loads. A geotechnical assessment interprets ground investigation data on soil strength, plasticity, groundwater and stability to inform foundation design under Eurocode 7. One rests on a building inspection, the other on trial pits and laboratory testing.
Which report is needed for a barn conversion?
A structural appraisal. Permitted development rights for agricultural building conversion do not extend to works amounting to rebuilding, so prior approval turns on whether the existing structure is capable of conversion. The appraisal should identify which elements are retained and confirm the frame and walls can take the loading the conversion adds.
Can one engineer do both?
Often, where the practice has both disciplines. The reports remain distinct, because they rest on different evidence and reach different conclusions. What matters is that the ground investigation is designed by someone competent to interpret it, and that the structural inspection is carried out on site rather than inferred from drawings.
Which costs more?
The geotechnical assessment, in almost every case. It carries plant costs for a rig or excavator, an engineer on site, utility clearance before any excavation, sometimes traffic management, laboratory analysis and interpretive reporting. A structural appraisal is a site visit and a report, unless opening up, access equipment or several months of crack monitoring are needed.
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.