Class Q structural works and what is reasonably necessary
Class Q permits windows, doors, roofs, walls, services and partial demolition, and nothing that amounts to a rebuild. How the reasonably necessary test is applied and what the engineer must show.
Class Q structural works are limited to what is reasonably necessary to convert an agricultural building into a dwelling, and that phrase does the heavy lifting. The right covers replacing windows, doors, roofs and exterior walls, putting in services, and partial demolition. It does not cover building a new structure inside or around the old one, which is why so many barn conversions fail at prior approval stage on structure alone.
Class Q sits in Part 3 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. The permitted building operations are in paragraph Q.1, and the limits there are the ones an engineer needs to read before drawing anything.
The building operations the class allows
Paragraph Q.1 permits the installation or replacement of windows, doors, roofs and exterior walls, the installation or replacement of water, drainage, electricity, gas and other services, and partial demolition, in each case to the extent reasonably necessary for the building to function as a dwellinghouse.
Read that list as a closed one. If a proposed operation is not on it, the right does not authorise it, whatever its merits. Underpinning, new foundations, a replacement frame and an entirely new floor slab are the usual casualties.
The 2024 amendments to Class Q changed the number of dwellings and the floorspace limits and introduced a narrow allowance for a single storey extension on existing hard surface. Those limits move, so work from the current consolidated text on legislation.gov.uk on the day you advise, not from a summary.
The reasonably necessary test
The test is not whether the works are sensible, or whether they produce a better house. It is whether they are reasonably necessary to let the existing building function as a dwelling.
Planning practice guidance puts the structural point directly: the right is not intended to include the construction of new structural elements, and it applies where the existing building is strong enough to take the loading that comes with the conversion works. That single sentence decides most cases. If the walls cannot carry a new roof, or the frame cannot carry insulated cladding, the works needed to make them capable go beyond conversion.
There is a second, less obvious limb. Works can be reasonably necessary in principle and still be too extensive in aggregate. A schedule that replaces every wall, the whole roof and the floor is, taken together, a rebuild, even though each item appears on the permitted list.
Why a bare frame usually fails
The leading authority is Hibbitt v Secretary of State for Communities and Local Government, decided in the High Court in 2016. The court was dealing with an open sided agricultural building with a steel frame, a roof and no meaningful walls, and it held that Class Q permits conversion rather than fresh erection. Where the operations needed are so extensive that what emerges is a new building using the old frame as scaffolding, the right does not apply.
That reasoning is indicative of how inspectors have approached the question since, rather than a rule that settles every case. But the practical consequence is consistent. A Dutch barn with a portal frame, a sheet roof and open sides is a hard case. A blockwork or brick agricultural building with a sound roof structure and existing openings is a straightforward one.
The distinction that matters is not age or appearance. It is whether the existing envelope and structure are doing the work, with the permitted operations filling gaps, or whether the permitted operations are doing all the work.
What the structural survey has to show
Commission a structural engineer, not a builder's opinion, and brief them on the legal test rather than on the design. The survey should cover:
- The primary structure as found, with frame type, spans, section sizes and condition.
- The foundations, including what was exposed and how, since an assumption here undermines everything above it.
- The walls and roof, their construction, and their present condition with photographs keyed to a plan.
- The loads arising from the proposed conversion, including insulation, new cladding, roof coverings, internal floors and any new openings.
- A clear conclusion on whether the existing structure can carry those loads without new structural elements.
That final conclusion is the document. Everything before it is evidence for it. An engineer who writes that the building is in good condition, without addressing the loading question, has not answered the point the authority will decide on.
Where the honest answer is that some strengthening is needed, say so and consider whether a full planning application is the better route. A refused prior approval followed by an application is slower and more expensive than going straight to the application.
Roof replacement, wall rebuilding and the limits
Replacing a roof is expressly permitted, and replacing exterior walls is expressly permitted. The friction comes from scale and from the dimensional limits.
Partial demolition is allowed to the extent reasonably necessary. Demolition of most of the structure followed by rebuilding on the same footprint is not partial in any ordinary sense, and describing it as such in a schedule of works tends to attract the refusal it deserves.
Where a wall is to be rebuilt, the application should explain why: a specific section is unsound, it is identified on the drawings, and the rebuilt element carries no more than the original did. Where a roof is to be replaced, the new covering weight should appear in the engineer's loading calculation. These are small points that turn a contested case into an uncontested one.
The site constraints sit alongside the structural test and are worth checking first, because they are absolute. Article 2(3) land, listed buildings and scheduled monuments are excluded outright, flood zone restrictions apply to both Class Q and Class MA, and an Article 4 direction can remove the right altogether in a defined area.
When an authority refuses on structural capability
A refusal on this ground usually says that the works proposed exceed those reasonably necessary, or that insufficient information has been submitted to demonstrate that the building is capable of conversion. Those are different failures with different answers.
If the refusal is about information, resubmit with the engineer's report done properly. There is no limit on making a further prior approval application, and the 56 day determination period starts again. If the refusal is about the substance, an appeal under section 78 is available, and the inspector will apply the same conversion test on the evidence in front of them.
Either way, get the evidence in with the application rather than after it. The authority decides on what it has at the end of the period, and an engineer's report emailed on day 50 rarely gets the attention it needs.
Putting the structural case with the application
Submit a schedule of works that maps every operation onto the categories in paragraph Q.1, with a column giving the reason each is reasonably necessary. Alongside it, submit existing and proposed elevations with the retained structure clearly distinguished from new work, ideally by hatch rather than by note.
A short supporting Planning Statement tying the schedule, the engineer's conclusion and the prior approval matters together helps, particularly where the authority has asked about flooding or highways as well. Keep it to the statutory matters. Design merit arguments do not assist under Class Q and can suggest the applicant has the wrong route in mind.
One financial point that surprises people: prior approval schemes can still be liable for the community infrastructure levy, so check the charging position before the notice is issued rather than after commencement.
A prior approval decision confirms that the specified matters are acceptable. It does not certify that everything you intend to build falls within the class. Where the margin is tight, a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that the works are lawful.
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.
Frequently asked questions
Can I put a new steel frame inside a barn under Class Q?
No. Planning practice guidance says the right does not extend to constructing new structural elements, and the existing building must be capable of taking the loading that comes with the conversion. A new frame is the clearest example of works falling outside the class, and authorities refuse prior approval on that basis regularly.
Does an open sided Dutch barn qualify for Class Q?
It is a hard case. In Hibbitt, decided in 2016, the High Court held that Class Q permits conversion rather than fresh erection, and an open sided steel framed building usually needs works so extensive that the result is a new building. Some have succeeded where the existing structure does carry the conversion, so the engineer's evidence decides it.
What should a Class Q structural survey contain?
A description of the primary structure, foundations, walls and roof as found, photographs keyed to a plan, the loads arising from the proposed conversion including insulation and new coverings, and an express conclusion on whether the existing structure can carry those loads without new structural elements. That conclusion is the part the authority decides on.
Can I replace the whole roof under Class Q?
Roof replacement is expressly permitted where it is reasonably necessary for the building to function as a dwelling. The difficulty arises when the replacement roof is heavier than the original and the walls or frame cannot carry it, because the strengthening needed then falls outside the class. Include the new covering weight in the loading calculation.
What happens if prior approval is refused on structural grounds?
If the refusal is for insufficient information, make a further prior approval application with a proper engineer's report, which restarts the 56 day period. If it is on the substance, appeal under section 78 of the Town and Country Planning Act 1990, where an inspector applies the same conversion test on the evidence submitted.
Does a prior approval notice confirm my works are lawful?
It confirms only that the specified prior approval matters are acceptable. It does not certify that every operation you intend falls within Class Q. Where the margin is tight, apply for a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990, which is the only formal way to confirm that proposed works are lawful.
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.