A Guide to Converting Existing Barns to Meet UK Building Regulations in 2025
Planning & Design Practice has mastered the use of National Class Q permitted development rights over the past 10 years and continues to be at the forefront of the use of the legislation since the latest changes to its format last year. Alongside the planning considerations of using this approach to create new homes the practice has also developed significant knowledge of the practical and technical hurdles of converting these buildings.
Whether working with stone, brick or steel portal frame barns each site and structure presents unique challenges and opportunities. To ensure compliance with current UK building regulations, barn conversions must meet strict standards for structural integrity, energy efficiency, and safety and these issues are carrying ever increasing weight with planning officers. This article explores the key considerations and construction methods required to meet these standards while retaining the character and charm of these rural structures.
Understanding UK Building Regulations
UK building regulations are designed to ensure that converted buildings are safe, energy-efficient, and suitable for habitation. Key areas of focus include:
- Structural Stability: The building must be sound and capable of supporting the proposed conversion.
- Thermal Performance: Insulation must meet or exceed current U-value requirements.
- Ventilation: Adequate airflow is essential to prevent damp and ensure a healthy living environment.
- Fire Safety: Fire-resistant materials, cavity barriers, and escape routes must be integrated into the design.
- Access and Facilities: The conversion must provide suitable access, an accessible WC facility and sufficient natural light.
Let’s delve into how these considerations can be addressed for different types of barns.
Stone Barn Conversions
Stone barns, with their thick, load-bearing walls and traditional charm, are often the first choice for conversion. However, meeting modern building standards requires careful planning.
Insulation and U-Values
Whist stone has significant thermal mass, it is generally a poor insulator on its own, so upgrading its thermal performance is critical. Current UK regulations set out a limiting U-Value for new build dwellings of 0.26 W/m²K or better. Achieving this value within the use of thick PIR insulation would be difficult. We would typically aim to avoid using this type of non-breathable insulation in traditional barn conversion, however we have explored some options for improving the thermal efficiency of stone buildings below. This could involve:
- Internal Wall Insulation (IWI):
- Use insulated plasterboard or rigid foam panels to line the interior walls.
- Ensure a breathable membrane is installed to prevent trapped moisture, otherwise this can lead to damp issues.
- Consider using natural materials like wood fibre with lime plaster or sheep’s wool insulation with timber studs for better breathability and environmental performance.
- External Wall Insulation (EWI):
- If allowed by planning constraints, EWI can be applied to the exterior. This involves attaching insulation boards to the outside and finishing with a render.
- Stone cladding can be applied to retain the original appearance.
- External insulation will need to take into account depths of soffits, whether any architectural features would be hidden etc
Structural Reinforcement
Older stone barns may need structural reinforcement to handle additional loads from insulation, roofing, and flooring. Methods include:
• Installing tie bars to improve lateral stability.
• Re-pointing with lime mortar to maintain breathability.
• Reinforcing foundations, if necessary, to accommodate modern floors but this needs to be done in such a way that does not go beyond the scope of Class Q.
Windows and Doors
Stone barns often have small openings, which can limit natural light. To meet regulations:
• Enlarge existing openings where possible, while maintaining structural integrity.
• Install double or triple-glazed units with thermally broken frames to improve energy efficiency.
• Windows will typically need trickle ventilation in line with Building Regulations
Fire Safety Considerations
• Stone walls offer inherent fire resistance, but internal finishes must comply with Part B of the building regulations.
• Ensure cavity barriers are installed in roof voids and wall cavities to prevent fire spread.
• Use non-combustible insulation materials to enhance fire safety.
Brick Barn Conversions
Brick barns, like stone barns, offer a great opportunity for conversion but again may require significant work to meet current standards.
Insulation and U-Values
Brick walls typically lack the thermal mass of stone, so insulation is critical. Brick built agricultural structures typically won’t benefit from a cavity wall construction and therefore options for insulating externally or internally will need to be explored. A critical consideration in these cases will be prevention of condensation. :
- Internal or External Insulation:
o Similar to stone barns, brick structures can benefit from IWI or EWI. However, attention must be paid to avoiding thermal bridging at junctions.
Addressing Damp
Brick barns are prone to rising damp and water ingress. Measures to mitigate this include:
• Installing a damp-proof course (DPC) or damp-proof membrane (DPM).
• Ensuring adequate drainage around the barn to prevent water pooling.
Roof Insulation
Brick barns often feature timber-framed roofs. Insulating the roof is vital to prevent heat loss, and breathable membranes can be used to maintain ventilation and prevent condensation.
Fire Safety Considerations
• Brick walls are non-combustible, but internal materials must meet fire safety requirements.
• Cavity barriers are essential in any insulated walls and roofs to prevent fire spread.
• Avoid the use of composite cladding with combustible cores; instead, opt for mineral-based materials.

Steel Portal Frame Barn Conversions
Steel portal frame barns, often clad with steel or asbestos sheeting, present unique challenges due to their lightweight construction and lack of thermal performance but can create stunning homes.
Insulation and Cladding
Meeting thermal requirements for steel-framed barns requires careful attention to insulation and condensation control. Current regulations mandate a U-value of 0.18 W/m²K for roofs and 0.26 W/m²K for walls. Solutions include:
- Non-Combustible Cladding:
o Replace existing cladding with non-combustible materials such as steel or aluminium panels with mineral wool cores but be mindful of the legislation and the need for the works to amount to conversion and not ‘fresh build’
o Avoid composite panels with combustible cores to comply with Part B fire safety regulations. - Internal Insulation:
o Add insulation inside the frame, using materials like mineral wool or rigid non-combustible boards.
o Install a vapor control layer to prevent condensation and protect the steel frame.
Addressing Asbestos
Many older barns feature asbestos cladding. Retaining asbestos is sometimes an option if it is in good condition. Encapsulation involves sealing the material with a protective coating to prevent fibres from becoming airborne. However, this approach must strictly adhere to UK Health and Safety Executive (HSE) guidelines to ensure there is no risk to health.
We would recommend removal and replacement of the asbestos cladding. If removal is necessary, it must be carried out by licensed professionals under controlled conditions. Replacement materials must be non-combustible and meet current building regulations.
Structural Considerations
Steel frames may need reinforcement to support additional loads from insulation, new cladding, and roofing materials. Engineers should assess:
• Load-bearing capacity of the frame.
• Anchoring and foundation adequacy.
• Compatibility with new materials.
Fire Safety Considerations
• Steel frames must be protected with intumescent paint or fire-resistant boards to achieve the required fire resistance.
• Install cavity barriers within wall and cladding systems to prevent fire spread.
• Ensure fire escape routes and emergency lighting are incorporated into the design.


Expanding the Footprint: The New Class Q Allowance of 200mm
Under the updated Class Q regulations, buildings undergoing conversion can now extend their footprint by up to 200mm beyond their existing roof and wall dimensions. While this increase may seem modest, it opens up significant opportunities for improving the building’s thermal performance and structural integrity without requiring new foundations. Here’s how this change can be effectively utilised:
Constructing a New External Wall Leaf
The additional 200mm can accommodate a new external wall leaf to improve insulation and weather resistance. Options include:
- Rainscreen Cladding Systems:
o Utilise helping hand brackets to attach lightweight, non-combustible cladding panels.
o This approach allows for a ventilated cavity between the cladding and insulation, reducing the risk of condensation. - Steel Frame Systems (SFS):
o Add a steel framework to support new cladding and insulation layers.
o SFS is lightweight and can be anchored to existing walls without the need for new foundations.
Addressing Foundations
Adding a new wall leaf may require adjustments to the building’s foundation width to support the additional load. Two potential solutions include:
- Improving Existing Foundations:
o If the existing foundation is shallow, it may be possible to excavate and extend its width.
o Reinforcement bars can be used to integrate the new section with the existing foundation. - Using Lightweight Materials:
o Opt for materials like insulated panels or aerated concrete blocks, which reduce the overall weight and minimise foundation stress.
Expanding the Roof by 200mm
Adding 200mm to the roof provides an opportunity to improve insulation and drainage. Key considerations include:
- New Roof Structure:
o A steel or timber framework may be required to extend the roof eaves and support new insulation layers. This can take a building beyond the scope of Class Q so it is strongly advised that you seek professional advice if this is required.
o Ensure that the extended roof is designed to prevent water ingress and maintain adequate overhangs. - Insulation Upgrades:
o Use rigid insulation boards or spray foam to achieve the required U-value of 0.18 W/m²K for roofs.
o Incorporate a breathable membrane to allow moisture to escape while preventing water penetration. - Drainage Improvements:
o Extending the roof allows for better integration of gutters and downpipes, ensuring effective water management.
Ventilated Cold Roof vs. Non-Ventilated Warm Roof
When upgrading or expanding the roof, it’s essential to choose the right insulation method. The two primary options are:
- Ventilated Cold Roof:
o Insulation is placed between the ceiling joists, leaving the roof space unheated.
o Requires adequate ventilation to prevent condensation, often achieved through vents in the eaves or ridge.
o While this method can be cost-effective, it is less efficient and carries a higher risk of thermal bridging and condensation. - Non-Ventilated Warm Roof:
o Insulation is placed above the roof deck, keeping the entire roof structure warm.
o This method eliminates the need for ventilation and minimises thermal bridging.
o Warm roofs are highly effective at preventing condensation and improving overall energy efficiency.
Why Choose a Warm Roof?
Warm roof construction is generally recommended for barn conversions due to its superior performance:
• Reduced Condensation Risk: By keeping the roof structure warm, condensation is less likely to form, protecting the building fabric.
• Improved Thermal Performance: Continuous insulation reduces heat loss and minimises cold spots.
• Longevity: Warm roofs are less prone to moisture-related issues, ensuring a longer lifespan.
Achieving a warm roof can involve:
• Adding rigid insulation boards above the roof deck.
• Using a waterproof membrane over the insulation layer.
• Finishing with durable, non-combustible roofing materials such as metal sheets or tiles.
At PDP we specialise in navigating the complex planning and technical requirements for barn conversions. Our expertise ensures that your project meets all regulatory standards while taking the detail of Class Q legislation into account. Over the coming months we will also be exploring the internal and external design possibilities of working with existing barns, that can make for beautiful and unique homes. If you need guidance on converting your barn, contact us today at enquiries@planningdesign.co.uk. We’re here to help turn your vision into reality.
David Symons, Senior Architect, Planning & Design Practice Ltd


