Bespoke is best

James Hulbert of Knauf Insulation explains how housebuilders can navigate their way to cost-effective part L 2021 compliance in England.

Part L was published in December and will bring England’s new build homes closer to net zero, and its pivotal role is reflected in the speed of implementation. The changes will come into force in June this year. Transitional arrangements allow for a brief grace period where homes with planning permission can be built to the old standard but after June 2023, all undeveloped plots (not sites) must comply with the new standard regardless of when planning permission was obtained. So, housebuilders must get to grips with the implications fast.

For many that will be easier said than done. Recent Industry Viewfinder research by Housebuilder & Developer revealed that over half of respondents didn’t understand what Part L’s 31% reduction in carbon would mean for their developments. Furthermore, 80% expressed concerns about the cost implications of compliance.

I want to reassure housebuilders that compliance can be both achievable and affordable.

Under the new regulations, CO2 emissions from new build homes must be 31% lower than current standards, which will be delivered by fabric improvements as well as low and zero carbon technologies.

But, Part L 2021 does not require a wholesale rethink of the building fabric. Housebuilders retain a large degree of freedom to find the most cost-effective mix of technology and fabric improvements that will achieve compliance.


The most important message is that the notional dwelling is just one example of how to achieve compliance for a typical home. It outlines a ‘recipe’ for fabric specification; U-values for the walls, roof, floors, windows and doors, as well as parameters for thermal bridging, air permeability and heating design.

It’s not the only way, or indeed the best way. Most housebuilders are unlikely to find it is the most cost-effective route to compliance.

The notional dwelling for Part L 2021 includes U-values of 0.11 W/m²K for the roof and 0.18 W/m²K for the walls. Adding more mineral wool insulation to the loft is an easy and extremely costeffective way of achieving a U-value as low as 0.09 W/m²K, so 0.11 is arguably an under-specification. Loft space is usually dead space, so the plot footprint won’t be affected.

By contrast, achieving 0.18W/m²K in cavity walls is more costly, and may require compromises. Housebuilders may find they need to adopt wider cavities (and therefore plot footprints).

In essence, following the notional dwelling to the letter could lead to unnecessary expense when compliance can be achieved in a smarter, more cost-effective way.


Using the SAP methodology allows housebuilders to determine the right whole-house recipe for their requirements. A recipe that finds an optimal balance between building design, high-performance insulation, and low-carbon services.

In many cases, the fabric performance required for compliance can be delivered cost-effectively by adding insulation to the roofs and floors. This means that existing wall insulation preferences and build-ups can be retained.

The good news is that compliance can be achieved more cost-effectively by taking a bespoke approach instead of using the notional dwelling.

So how can it be done? Consider, a three-bedroom semi-detached house with two wet rooms and a gas combi boiler. Instead of the 0.18W/m²K wall U-value, compliance can be achieved by increasing floor and roof insulation, and improving the air tightness of the building, alongside low-cost wastewater heat recovery (WWHR). Coupling this with a simple ventilation system means some rooftop solar PV will be required, but even this can be avoided using upgraded mechanical ventilation with heat recovery.

In both cases, it’s possible to retain a 100 mm non-combustible fully filled cavity.

Ultimately, you need to comply with Part L, but don’t feel forced into following the notional dwelling route. There are better options available by going bespoke. If you don’t want to work with an external SAP assessor, then the right insulation supplier will have whole house recipes available to share.

James Hulbert is head of housing at Knauf Insulation.