Class Q or a full application for a barn conversion
Class Q is faster and narrower. A full application is slower, costs more in evidence and carries mandatory biodiversity net gain, but it lets you design the building the barn deserves.
Whether to use class Q or full planning for a barn conversion comes down to three things: how many units you want, how much you need to change the building, and whether the site qualifies at all. Class Q of Part 3 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 gives a prior approval route to convert agricultural buildings to dwellings, with limits on numbers and floorspace and a narrow list of matters the council may consider. A full application has no such limits and no such protection.
The class was amended in 2024, changing the unit numbers, the floorspace limits and some of the conditions. Read the current text rather than a summary written before then, including this one.
What each route allows for units and floorspace
Class Q caps what you can do. It permits a defined maximum number of dwellings from a single established agricultural unit, with a maximum floorspace for each dwelling and a maximum total across the unit. Those figures changed in the 2024 amendments, so check them against the Order as it now stands.
A full application has no numerical ceiling. What limits it is policy: whether the local plan supports residential conversion in the countryside, whether the location is sustainable, whether the scale is appropriate to the building and its setting.
That produces a simple rule of thumb. If the scheme fits inside the class, the class is usually quicker. If it does not, arguing for more units through prior approval is not possible, because the limits are jurisdictional rather than matters of judgement.
Design freedom and external alterations
Class Q permits only building operations reasonably necessary for the building to function as a dwelling. That covers the installation or replacement of windows, doors, roofs and exterior walls, and services such as water, drainage, electricity and gas, together with partial demolition to the extent reasonably necessary.
What it has never permitted is a scheme that is, in substance, a rebuild. The building has to be structurally capable of the conversion as it stands, and the appeal record is full of cases where a proposal failed because the works went beyond conversion into reconstruction. A structural appraisal by an engineer, confirming that the frame and walls can take the loads without substantial rebuilding, is the single most useful document in a Class Q submission.
A full application lets you do what the building actually needs. New openings in different positions, a linking element, a change of roof form, an extension, a rebuilt gable. For a barn with real character, that freedom often produces a better building than a conversion squeezed into the class limits, and it lets you keep the agricultural reading of the structure rather than domesticating it awkwardly.
Where design quality is the argument, a Planning Statement that sets out the building's significance, the conversion strategy and the policy case does far more work than under prior approval, where most of it would be irrelevant.
Matters the council may consider under each route
This is the heart of the comparison.
| Route | What the council can assess |
|---|---|
| Class Q prior approval | Transport and highways, noise, contamination, flooding, whether the location makes the change impractical or undesirable, design and external appearance, natural light to habitable rooms |
| Full application | Everything material, including landscape, heritage, ecology, sustainability of location, design, amenity and local plan policy |
Under Class Q the council cannot refuse because it dislikes new houses in the countryside. Under a full application it can, and in many rural authorities it will unless the local plan supports conversion.
The reverse is also true. Prior approval refusals frequently turn on the impracticality or undesirability limb, which gives officers more latitude than applicants expect, particularly on isolated buildings served by a poor track.
Biodiversity net gain and the ecology difference
This is the change that has altered the arithmetic most since 2024, and it is regularly missed.
Mandatory biodiversity net gain under the Environment Act 2021 attaches to planning permissions granted on an application. Permitted development, including a Class Q prior approval, is outside that regime. So a full application for a barn conversion will normally need a biodiversity gain plan approved by the council before development, with a 10 per cent uplift secured for 30 years, while the equivalent Class Q scheme will not.
That difference is real money and real programme. A Biodiversity Net Gain Assessment uses the statutory metric and must be completed by a competent person, and the gain plan is approved by the local planning authority. Exemptions exist, including for self-build and custom build and for development below the small site threshold, so check whether one applies before assuming the cost.
Protected species are a different matter and apply to both routes. Barns are among the most reliable bat roosts in the countryside, and an emergence survey can only be done in the warmer months. A Preliminary Ecological Appraisal should be the first thing commissioned on either route, since a survey requirement discovered in October costs a season regardless of which application you make. Where bats are present, surveys and any licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the habitats regulations, and no AI-assisted draft changes that.
Timescales, fees and appeal rights
Prior approval for Class Q is determined within 56 days of a valid application, and the fee is lower than for a full application, though fees are indexed annually so check the current schedule.
A full application for a minor development is determined in eight weeks, extendable by agreement, and in practice rural conversions often take longer because of consultee responses. The evidence base is larger, which adds cost before you even apply.
Both routes carry a right of appeal to the Planning Inspectorate against refusal or against conditions. The difference is what an inspector can consider, which mirrors the table above.
Sites where Class Q is unavailable
Check eligibility before spending anything. Class Q does not apply to listed buildings or land within their curtilage, to scheduled monuments, or on land designated as a National Park, a National Landscape, the Broads, a World Heritage Site or a conservation area. Safety hazard areas, military explosives storage areas and sites of special scientific interest are also excluded.
The building itself must have been in agricultural use on the qualifying date set by the class, and there are conditions about existing access. These were among the provisions changed in 2024, so verify each one against the current text and gather the evidence for the use history early. Business records, subsidy claims, photographs and statutory declarations all help.
Where the site fails on any of these, the comparison ends and a full application is the only route.
Using a full application to improve on a Class Q consent
The most effective tactic in rural conversion work is to obtain the prior approval first and then apply for something better.
An extant Class Q prior approval is a real fallback. It shows that a dwelling can lawfully appear in that location, which changes the question from whether there should be a house there to what kind of house it should be. Inspectors have given weight to that reasoning where the fallback is real and likely to be implemented.
Two conditions make the argument work. The prior approval has to be capable of implementation, which means the structural evidence has to stand up. And the full application has to be demonstrably better in landscape and design terms than the fallback, not merely larger.
Where the building sits prominently in open countryside, a proportionate Landscape and Visual Impact Assessment comparing the fallback with the proposal is the document that makes the comparison visible to the officer.
Choosing between them at feasibility stage
Work through it in this order. Confirm eligibility for Class Q. Get a structural appraisal. Get an ecology walkover. Establish the use history. Then decide.
Choose Class Q where the scheme fits the limits, the building is sound, the location is unremarkable and speed matters. Choose a full application where the building deserves better than the class allows, where the numbers exceed the caps, where the site is ineligible, or where the local plan actively supports conversion. And consider taking both, in sequence, where the fallback would strengthen the case.
This is general guidance, not legal advice. Class Q has been amended repeatedly, so check the current text of the Order and confirm the position with your local planning authority or a qualified professional before relying on it.
Work out which evidence each route needs before you commit. Enter your postcode on PlanScope to see what a barn conversion is likely to require.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is Class Q faster than a full application?
Usually. Prior approval under Class Q is determined within 56 days and the fee is lower, while a minor full application runs to eight weeks and often longer once consultees respond. The larger difference is the evidence base, which is smaller for prior approval because the council can only assess a defined list of matters.
Does biodiversity net gain apply to a Class Q conversion?
Mandatory biodiversity net gain attaches to planning permissions granted on an application, so permitted development including Class Q prior approval falls outside it. A full application for the same barn will normally need a biodiversity gain plan with a 10 per cent uplift secured for 30 years, unless an exemption applies.
Can I extend a barn under Class Q?
The class permits only building operations reasonably necessary for the building to function as a dwelling, and it has never allowed a scheme that amounts to reconstruction. Some limited additions were introduced by amendment in 2024, so check the current text. For anything beyond that, a full application is the route.
What if my barn is in a conservation area or a National Landscape?
Class Q is not available on that land, nor for listed buildings and land in their curtilage, scheduled monuments, sites of special scientific interest, safety hazard areas or military explosives storage areas. Where the site is excluded the comparison ends and a full planning application is the only route available.
Should I get prior approval first and then apply for something better?
It is a common and effective approach. A live Class Q prior approval is a real fallback showing that a dwelling can lawfully appear on the site, which shifts the argument to what kind of house it should be. The fallback has to be capable of implementation in practice, so the structural evidence must stand up.
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.