Drainage Strategy and Design for Two Rural New Builds: A Mid Devon Case Study

FPS Environmental  | July 2026

Getting planning permission for new homes on a farm is rarely straightforward. This case study follows a farm holding in Mid Devon where two redundant agricultural barns were eventually replaced with two new houses, and explains the role drainage played at every stage.

The project moved from Class Q barn conversion approvals, through a refused first application, to full planning permission for two new builds. At each step, the council needed evidence that surface water and foul drainage could be managed safely and would not increase flood risk.

If you are a landowner, self-builder or developer considering a similar rural scheme, or an architect or planning consultant supporting one, this project shows what to expect and when drainage input is needed.

What is Class Q

Class Q is a permitted development right in England that lets you convert an agricultural building, like a barn, into a house without needing full planning permission. Instead of a normal application, you submit a “prior approval” application, where the council can only assess specific matters: things like flooding, contamination, noise, highways access, and whether the building’s location makes it practical to live in.

The building must genuinely be convertible; you can work with the existing structure but can’t rebuild it. There are also limits on size and the number of homes you can create.

Once Class Q approval establishes the barn can lawfully become a home, you can argue to the council that a well-designed new build is a better outcome than converting a tired agricultural shed, which is how this project went from barn conversions to full permission for two new houses.

The project: from Class Q barn conversions to new builds

The site contained two redundant agricultural barns within a working farm holding. The owners’ long-term aim was two new houses, but the route there took several stages over about three years:

• Class Q prior approvals: permitted development consent to convert each barn to a dwelling, establishing the principle of residential use on the site.
• Householder application: an extension to the main farmhouse, which helped demonstrate that the proposed new dwellings would remain subordinate to it.
• First full application refused: the initial proposal to replace the barns with two new builds was not accepted.
• Renewed Class Q consent: a fresh prior approval for conversion to two dwellings strengthened the fallback position.
• Full permission granted: a revised application for two new dwellings, access and associated works was approved, followed by a non-material amendment and discharge of conditions.

This is a classic Class Q “fallback” strategy. Because the barns could lawfully become homes through conversion, the council could weigh that fallback when considering whether well-designed new buildings were the better outcome.

Why drainage mattered at every stage

Class Q prior approval

Flooding and contamination are express prior approval matters under Class Q. Even at this early stage, the applicant must show the site is suitable for residential use, including how roof and yard drainage will be dealt with once the building changes use.

Full planning application

For the full applications, Mid Devon’s validation requirements include a surface water drainage strategy. The Lead Local Flood Authority (Devon County Council) is consulted, and a scheme without credible drainage evidence risks objection, delay or refusal.

The strategy must demonstrate that runoff from the new dwellings, driveways and access will be managed in line with national policy, following the drainage hierarchy and without increasing flood risk elsewhere.

Planning conditions

Permission was granted subject to conditions, including drainage. Discharging these required detailed drainage design: calculations, layouts and construction details that the council could approve before and during build.

The drainage design

FPS Environmental’s Head of Flood and Civils led the detailed drainage design for the new dwellings.

Surface water

The site was entirely off grid, so the design followed the SuDS hierarchy and prioritised infiltration:

• On-site soakaway testing confirmed the ground could accept infiltration
• Underground crate soakaways were sized to store and infiltrate runoff from roofs and hardstanding
• The result: no increase in flow rates leaving the site, with everything managed within the plot

Foul water

With no public sewer available, each dwelling was served by a sewage treatment tank discharging to a drainage field, designed in line with the Environment Agency’s general binding rules and Building Regulations Part H.

Locating the tanks and drainage fields early mattered: they need separation from buildings, boundaries and the soakaways, and that affects plot layout.

Access and highways

The permission also included a new vehicular access. Visibility splays were agreed through direct consultation with the county highways authority, with an access plan and supporting explanation submitted as part of the application.

Lessons for rural development projects

• Bring drainage in early. Drainage evidence shaped the outcome at prior approval, full application and condition stages. Late drainage input is a common cause of validation delays and refusals.
• A refusal is not the end. The first full application failed, but a strengthened fallback position and a revised, well-evidenced scheme succeeded a year later.
• Class Q does not remove drainage requirements. Prior approval still tests flood risk, and Building Regulations still apply to the conversion or new build.
• Detailed design pays for itself. Approvable calculations and drawings meant conditions could be discharged without redesign, keeping the build programme moving.

Conclusion

This project shows that drainage is not a box-ticking exercise on rural schemes. It is part of the planning strategy from the first Class Q notification through to the final discharge of conditions.

If you are planning a barn conversion, replacement dwelling or small rural development, commission your surface water drainage strategy alongside your architectural design, not after it.

FPS Environmental provides drainage strategies, detailed drainage design, soakaway design, SuDS design and flood risk assessments for projects of this kind across the UK. See more of our work at fpsenvironmental.co.uk/case-studies.