Class Q: proving the building is structurally sound enough

Class Q permits the conversion of an agricultural building into dwellings. It does not permit its replacement, and that single distinction is what a Class Q structural survey exists to address. The building has to be capable of conversion as it stands, strong enough to carry the loads the new works impose, and the survey has to show it rather than assert it. A steel portal frame with open sides and no walls is not a building capable of conversion, however neat the proposed floor plan looks.

What Class Q permits

Class Q sits in Part 3 of Schedule 2 to the General Permitted Development Order 2015 and allows a change of use of an agricultural building to dwellinghouses, together with building works reasonably necessary for that conversion, subject to prior approval.

The limits were raised with effect from 21 May 2024. The right now extends to up to 10 dwellings, each no larger than 150 m², with a cumulative total of 1,000 m² of floorspace changing use. The qualifying date moved too, so the building has to have been part of an established agricultural unit on 24 July 2023, with an alternative route where the building came out of agricultural use before then.

Class Q is unavailable on article 2(3) land, which takes in National Parks, National Landscapes, the Broads, World Heritage Sites and conservation areas, and it does not apply to listed buildings, scheduled monuments, sites of special scientific interest, safety hazard areas or military explosives storage areas. Read the current restrictions rather than any summary, including this one, because the class has been amended repeatedly.

The reasonably necessary test

The permitted building works are those reasonably necessary to convert the building, and the order lists the kinds of work contemplated: partial demolition, the installation or replacement of windows, doors, roofs and exterior walls, and services for water, drainage, electricity and gas.

That list is broader than people expect, and it is still bounded. Planning Practice Guidance makes clear that the intention is not to allow rebuilding, and that the building should be structurally strong enough to take the loading which comes with the external works needed for residential use.

The courts have held to that line. The leading case on Class Q structural works concerned a steel frame with a corrugated roof and no substantial walls, where the proposal required so much new structure that the result would have been a new building wearing the old one as a shell. That is the test in practice: would a reasonable observer say you converted a building, or built one. The detail of what falls inside the permitted works is set out in Class Q structural works and what is reasonably necessary.

What the structural survey has to demonstrate

A survey that says the building "appears structurally sound" is worthless. The authority needs to see the load path from the new roof and the new walls down to the ground, and a judgement about each element.

A survey that does the job covers:

1. The existing structural form, with the frame type, spans, member sizes and connections recorded.
2. The condition of each element, including corrosion to steelwork, decay to timber, cracking and spalling to masonry, and the state of any cladding rails.
3. The foundations, with trial holes where the loading is changing, and an assessment of bearing capacity.
4. The floor slab, its thickness and condition, and whether it can carry the new internal walls.
5. The loads the conversion will impose, quantified, and a comparison with the capacity of the existing structure.
6. A schedule of works separating repair and strengthening of existing elements from the introduction of new structure.
7. A conclusion stating whether the building can be converted without works going beyond those reasonably necessary.

Item six is the one the officer reads first. Where the schedule shows new foundations, new columns and a new roof structure, the conclusion writes itself and not in your favour.

A Structural Survey for this purpose should be prepared by a suitably qualified engineer. Councils expect that, and an appeal on a Class Q refusal with a survey by someone without structural competence is a difficult appeal.

Conversion against rebuilding

The pattern that fails is familiar. An open sided Dutch barn or a bare portal frame, a proposal to insert a complete new timber frame inside it, clad it, roof it and floor it, and a survey concluding that the original frame will carry the roof.

The difficulty is that the original frame usually carries almost nothing once the new structure is in, which makes the retained frame decorative. Where the honest assessment is that the building cannot be converted, two routes remain. Apply for full planning permission for a replacement building in the countryside, which is a much harder policy case, or reduce the ambition so the existing structure does real work.

Modest buildings sometimes convert better than large ones. A former milking parlour with solid blockwork walls, a sound slab and a replaceable roof is a far better Class Q candidate than a 1980s steel shed twice its size.

The other prior approval matters

Structure is one issue among several. On a Class Q application the authority may also consider transport and highways impacts, noise impacts, contamination risks, flooding risks, whether the location or siting makes residential development impractical or undesirable, the design or external appearance of the building, and the provision of adequate natural light in all habitable rooms.

The impractical or undesirable test is wider than many applicants assume and is a common refusal ground. A building in the middle of a working yard, with a slurry lagoon 40 metres away and a silage clamp opposite, is a poor candidate regardless of its structure. Evidence of continuing agricultural use was also the subject of the separate test addressed in Class Q: proving the building was in agricultural use.

Sequencing the survey before the application

Commission the structural survey before the architect fixes the layout, not afterwards. The order matters because the survey tells you how many openings the walls can take, whether the roof can be replaced on the existing frame, and whether the slab needs breaking out, and each of those shapes the plan.

A workable sequence runs: confirm the exclusions and the qualifying date, obtain the structural survey, agree the schedule of works with the engineer, design the conversion around what the structure allows, then prepare the prior approval submission with the survey and schedule as appendices and a short Planning Statement tying them to the Class Q tests.

And if the engineer tells you the frame needs replacing, listen to them rather than looking for a second engineer.

This is general guidance, not legal advice. 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. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which documents a barn conversion is likely to need.