Structural appraisal content for a barn conversion

A Class Q prior approval stands or falls on whether the existing building can take the conversion works without being rebuilt. The barn conversion structural appraisal is the document that answers it, and the question it has to settle is narrow: is the structure strong enough to support the new roof, walls, floors and openings, using only the building operations that Class Q permits.

Councils refuse prior approval on this point more often than on any other, usually because the appraisal describes the barn's condition without ever saying whether it can carry the proposal.

What the appraisal has to demonstrate

Class Q permits a conversion, not a replacement. The building operations allowed are limited to installing or replacing windows, doors, roofs and exterior walls, providing services, and partial demolition reasonably necessary to carry the conversion out.

The High Court considered where that line falls in Hibbitt v Secretary of State for Communities and Local Government, which concerned a steel framed agricultural building with open sides. The court held that a scheme leaving little more than the frame, with new walls and roof built around it, went beyond conversion and amounted to a fresh building. A structural appraisal therefore has to show that the existing structure does the work, rather than acting as a template for something new.

In practice the appraisal must reach a stated conclusion: that the building is structurally capable of the proposed conversion, with any remedial works falling inside the permitted operations. A report that stops at describing the barn as being in fair condition has answered a different question.

Assessing the existing frame and walls

The assessment differs by construction type, and the honest ones say so.

Steel portal frames are usually the easiest case, because the frame is designed for loading and the conversion adds insulated panels rather than mass. The engineer checks section sizes, corrosion at the base of stanchions, haunch connections, and whether the frame was designed for the original light sheeting loads alone.

Traditional masonry barns bring different questions: wall thickness and bonding, lime or cement mortar, lateral restraint where floors were never present, the condition of the wall head, and whether existing openings can be enlarged without rebuilding piers. Timber framed and cruck barns need each member assessed for decay and insect attack, with connections checked individually.

Say what was measured and where. A sentence recording plumb readings on each elevation and crack widths at specific locations carries more weight than a general statement that the walls appear sound.

Foundations and their adequacy

Foundations are the gap in most appraisals. Agricultural buildings were often built on pad bases sized for a light frame, on strip footings a few hundred millimetres deep, or on nothing identifiable at all.

Conversion changes the loading. New wall constructions, upper floors, tiled rather than sheeted roofs and internal partitions all add weight, and residential use brings heating, drainage and a damp proof course that the original never had.

Trial pits at representative locations are the only reliable answer. The appraisal should record depth, width, bearing stratum and condition for each pit, with photographs and a scale rule in shot. Where underpinning or new pads are needed, say whether those works sit inside the permitted operations, because extensive foundation work is one of the routes by which a Class Q scheme becomes a rebuild. A geotechnical assessment covers the ground conditions those foundations bear on.

Distinguishing repair from rebuilding

This is where appraisals earn their fee. The engineer should list each defect and the intervention it needs, then classify it.

Repointing, localised stitching of cracks, replacing a decayed wall plate, inserting a new lintel over an existing opening and rebuilding a gable apex are ordinarily repairs. Taking down and rebuilding a full elevation, replacing more than a limited proportion of the frame, or building new walls outside the existing line are not.

A schedule format works well: element, observed defect, proposed intervention, whether it falls within the permitted operations, and the structural justification. Officers read that table first.

Where a barn is partly collapsed, be honest in the appraisal about which parts are beyond repair. A scheme that quietly proposes to rebuild a fallen elevation will be caught, and the refusal is harder to overturn than an application that acknowledges the limitation and reduces the footprint.

Photographs and opening-up evidence

Prior approval is decided on paper by an officer who may visit once. Photographic evidence does most of the persuading.

What to include:

1. Every elevation, square on, with a scale reference.
2. Internal views of the frame, roof structure and wall head.
3. Close ups of each defect referred to in the schedule.
4. Trial pit photographs showing depth and bearing stratum.
5. Any opening-up undertaken, with the location marked on a plan.

Date the photographs and identify the viewpoint on a key plan. Where access was restricted, say what could not be inspected and what assumption was made instead, because an unstated assumption is what an appeal inspector will fasten on.

Scale bars help more than people expect. An officer comparing a photograph with an elevation drawing needs something in the frame to measure against, and a folding rule against a wall head settles an argument about thickness in one image.

Author competence and indemnity

Appraisals are prepared by chartered structural or civil engineers with experience of existing buildings. Agricultural structures are a specialism of their own, and an engineer whose practice is new build design will produce a report that reads as one.

Check professional indemnity cover before appointing, and check that it will remain in place for long enough to matter. These policies work on a claims made basis, so cover has to exist when a claim is brought rather than when the report was written.

A structural survey drafted on PlanScope gives the reporting structure, and the engineer supplies the judgment, the measurements and the calculations. Councils expect a suitably qualified author for this work, and professional review before submission is strongly advised.

How the appraisal supports the application

Submit it as a standalone document with its own drawings, not as an appendix buried in the design statement. The officer assessing structural capability is often a different person from the one assessing siting and design.

Timing is worth planning too. Commission the appraisal before the architect fixes the openings, because the engineer will often move a window a metre to avoid rebuilding a pier, and that is a free change at sketch stage.

Cross reference it properly. The proposed plans should show which elements are retained, which are removed and which are new, using a consistent colour or hatching convention that matches the appraisal schedule. Where the scheme has changed since the appraisal was written, get an addendum rather than submitting a report that describes a different building.

And note the wider position: the 2024 amendments changed Class Q limits on unit numbers, floorspace and extensions, so check the current text of the Order before relying on any summary of it, including this one.

Browse the report types a barn conversion may need and draft the supporting documents on PlanScope before you brief the engineer.

This is general guidance, not legal advice, and a Lawful Development Certificate is the only formal way to confirm that works are lawful. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.