Client: Swift Unlimited

Brief: Advising on Building Regulations compliance for heated garden rooms, including when SBEM calculations are required

Swift Unlimited had designed two bespoke outdoor buildings: a garden room for a private golf simulator and gym in Southport, and another in Reading to house a hydrotherapy room for a family member with additional needs. The Cheshire-based firm, which builds the rooms using structural insulated panels (SIPs), called on Murton & Co for help after a Building Control Officer flagged up the need for both heated, powered buildings to demonstrate compliance with Part L of Building Regulations – determining that a SBEM (Simplified Building Energy Model) assessment was required.

Detached/stand-alone buildings with an internal footprint exceeding 50 square metres generally require Building Control approval, especially if heated or intended for regular use. Where a garden room is heated and used in a way that could be interpreted as non-domestic, for example, as a gym or treatment room, and the floor area exceeds 50 square metres, Building Control may require SBEM to demonstrate energy compliance under Part L.

“These garden rooms are often for very specific uses and aren’t commercial buildings, they’re domestic spaces with occasional use,”

says Swift’s Operations Director Matt Leighton.

Matthew Leighton – Operations Director at Swift Unlimited

“But despite that, the current Building Regulations don’t always reflect their real-world purpose. Instead, these two buildings were treated – on paper – as if they were public gyms or clinics.

“It means having to install extra insulation, more complex heating and ventilation setups and, ultimately, the result is higher costs to meet the same standards you’d expect for a commercial facility,” he adds. “That’s a big ask for what’s often a simple, private space in someone’s garden.”

The Part L calculation methodology treats buildings based on their use classification. If the intended use is considered non-domestic – regardless of the owner’s intentions – then SBEM may be required.

“These are low impact buildings, but due to the SBEM methodology, they can end up being assessed in the same way as a facility with paying customers,”

explains Jonny Murton-Lavelle.

“This is where homeowners and manufacturers could be caught out by the unexpected cost and complexity of compliance.”

While it can be challenging to meet the energy efficiency targets under SBEM, demonstrating good thermal performance, including low U-values and airtight construction, remains the cornerstone of compliance. Buildings must also consider the efficiency of their heating systems, and where gas or high-carbon systems are used, the rest of the design might need to compensate to achieve an acceptable performance level.

If a similar building were classified as a garden annexe or used as a separate dwelling, the SAP (Standard Assessment Procedure) methodology would apply instead – but this brings other challenges. SAP assessments require an overheating risk analysis under Approved Document O, Air Tightness Test and potentially a calculation of the Thermal Bridges which adds further time, cost and technical input to what is often a simple build.

In the Southport case, the building initially didn’t meet SBEM standards using the original specification.

“The options then are to improve the fabric performance, which might mean increasing wall thickness, improving the windows, or enhancing air tightness,”

says Matt.

“For us, increasing the panel thickness was the most straightforward route.”

Depending on the original spec, moving from 100mm panels to 150mm or even 172mm can add anything from £2,000 to £5,000+ depending on room size, cladding type, and the need for other changes such as foundations or roof adjustments.

To streamline future projects, Swift Unlimited worked with Jonny to establish a compliance blueprint, reviewing the company’s standard construction specification and calculating performance against Part L benchmarks. This allows the team to price projects accurately from the outset and understand whether SAP or SBEM is likely to apply.

“Jonny has helped make the process more transparent,”

says Matt.

“It enables us to present plans to a client and be more specific about the cost, so we can set expectations earlier and avoid having to make revisions later on, or worse – on site.”

There are doubtless other companies making bespoke garden rooms who haven’t considered these issues, says Matt.

“They might use thinner panels and sell the room unheated – like a glorified shed – to avoid Building Control. But if a client later adds a heater and uses it regularly, it could technically fall under SBEM. At that point, they’re at risk of non-compliance.”

In contrast, Swift Unlimited is confident that its buildings will meet the required standards and won’t trigger unexpected delays or expenses. The firm plans to collaborate with Murton & Co on future projects as Matt values Jonny’s integrity, experience and responsiveness.

“Working closely with Murton & Co from the early stages of a build has helped us plan the right compliance route from day one and avoid clients being caught out by regulations that don’t necessarily match their actual use of the building,”

adds Matt.

“Jonny gets what Building Control require and gets what we do. His tailored approach to integrate the two to achieve compliance is absolutely spot on.”