Structural evidence for a Class Q barn conversion

A Class Q structural report has one job: to show that the barn in front of you can be converted, rather than rebuilt. Prior approval under Class Q of Part 3 of Schedule 2 to the General Permitted Development Order 2015 allows the change of use of an agricultural building to a dwelling, plus the building operations reasonably necessary to make that work. If the honest answer is that the frame needs replacing and new foundations are going in, the right is not available and no amount of drawing will fix it.

That is why the structural evidence usually decides the outcome before the design does. Councils refuse Class Q prior approvals on structural grounds regularly, and the reasoning is almost always the same: the works proposed go beyond conversion.

The line between conversion and rebuilding

The limits on building operations sit in Class Q.1(i). Works are confined to the installation or replacement of windows, doors, roofs and exterior walls, to services to the extent reasonably necessary for the building to function as a dwelling, and to partial demolition to the extent reasonably necessary to carry those works out. Nothing in that list authorises a new frame or new foundations.

The High Court considered this in Hibbitt v Secretary of State for Communities and Local Government in 2016. The building there was an open-sided steel portal frame with a roof, and the court held that Class Q does not extend to what is in substance the creation of a fresh building using the old frame as scaffolding. The test is whether the operation can fairly be described as a conversion of an existing building.

Read that against the barn you have. A blockwork or brick agricultural building with a sound roof structure is usually convertible. A Dutch barn with no walls, a pole barn, or a frame that has been standing open to the weather for twenty years usually is not, whatever its footprint might allow.

What the structural engineer needs to establish

The report is a structural appraisal of an existing building against a proposed residential loading and enclosure, written by someone competent to do it. Councils normally expect a chartered structural or civil engineer, and professional review of any draft is strongly advised because the conclusion is the thing officers will weigh.

It should answer four questions in plain terms:

1. Is the primary structure, meaning foundations, frame or loadbearing walls and roof structure, capable of supporting the proposed use without replacement.
2. Which elements need repair, and does that repair fall inside the Class Q.1(i) list.
3. What is the sequence of works, and does any stage require the frame or walls to be removed and reinstated.
4. What residual risk remains, and how would it be managed on site.

An appraisal that says "the building is structurally sound" and stops there will not carry an officer. One that sets out member sizes, condition, spans, observed defects and a stated conclusion on convertibility will.

Foundations, frame and envelope assessed separately

Take the elements apart, because they fail differently and the planning consequence differs too.

Foundations are the common weak point in older agricultural buildings, which were often built on minimal or no formal foundations with a slab poured later. Trial holes are worth the cost. If the report concludes that underpinning or new pads are required across the building, you are outside Class Q, and a full planning application for replacement is the more honest route.

The frame or loadbearing walls carry the argument. For masonry, record wall thickness, mortar condition, bulging, cracking and lintel condition. For steel or timber frames, record section sizes, corrosion or decay at the base of stanchions and posts, and connection condition. A frame that needs plating or localised strengthening is usually still a frame being retained. A frame where half the stanchions are replaced is a new frame.

The envelope, meaning walls, roof covering and openings, is where Class Q gives the most latitude. Replacement of exterior walls and roofs is expressly permitted, so a new roof covering, new insulated walls behind the existing skin and new openings are not the problem. The problem arises when the existing walls are structural and are being taken down to be rebuilt.

That table is a working guide rather than a rule. Every appeal on this point turns on the facts of the building.

The rebuild line and how officers test it

Case officers look for two things. First, whether the submitted drawings show elements being removed that are not on the Q.1(i) list. Second, whether the structural report and the drawings agree. A report saying the frame is sound sits badly next to an elevation drawing showing a new steel frame in a different bay rhythm.

They also look at what the building is like now, in photographs. If a building has no walls, an officer will ask how the proposed walls are supported, and the answer will often reveal new structure. Deal with that in the report rather than leaving it to be found.

Where a building has a partial collapse, say the same thing openly and explain what is being retained. Attempting to present a collapsed structure as convertible is the fastest way to a refusal and a recorded planning history you then have to live with.

Photographic and measured evidence to include

Prior approval applications are decided on paper by an officer who may never visit. The evidence pack does the work of a site visit.

• A measured survey of the existing building: plans, sections and all elevations at a stated scale.
• Dated photographs of every elevation, the interior, the roof structure from below, and the base of each stanchion or a representative sample.
• Close photographs of defects, with a scale rule in frame.
• Trial hole logs and photographs where foundations have been investigated.
• A schedule of retained and replaced elements keyed to the drawings.
• A short statement on how the works fit the Q.1(i) description.

Keep the photograph metadata intact, and take the set on one day so the record is internally consistent. Where the building sits in a sensitive landscape, the same visit is the moment to gather material for a Landscape and Visual Impact Assessment if the design or external appearance is likely to be contested under Q.2.

Responding to a refusal on structural grounds

Three routes open up, and they suit different situations.

Resubmission with better evidence works where the building can in fact be converted and the first attempt was thin. A proper appraisal with trial holes, a schedule of retained elements and revised drawings that remove the offending works can turn a refusal round.

An appeal under section 78 works where the disagreement is about judgement rather than fact. The Planning Inspectorate decides Class Q appeals on written representations in most cases, and structural evidence carries real weight there. Prepare for the inspector to read Hibbitt.

A full planning application for a replacement dwelling is the route where the honest structural answer is that the building cannot be converted. It is a harder policy case in the open countryside, and it needs a proper Planning Statement addressing the relevant countryside and design policies in the adopted local plan and the current National Planning Policy Framework. Many barn schemes end up here, and the ones that succeed are those that stopped arguing about a frame nobody could save. Our guide to farm diversification buildings covers the adjacent question of what else that footprint could lawfully become.

One honest uncertainty: the amount of repair an inspector will treat as conversion varies, and there is no published percentage. Anyone who quotes one is guessing. What holds across decisions is the direction of the question, which is whether the existing building is doing the structural work afterwards.

Enter your postcode on PlanScope to see which reports your site is likely to need, including a structural survey that a chartered engineer can then review and sign off through the professional directory.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A structural survey is a report councils expect from a suitably qualified author, so professional input is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.