Bath council’s testbed net zero planning policy shows promise

Researchers from the University of Bath’s Department of Architecture & Civil Engineering have carried out a review of the first ever Net Zero Carbon construction policy to be implemented at a local authority.

Working in partnership with Chapter2 Architects and the South West Net Zero Hub, researchers studied every planning application made to Bath & North East Somerset Council in the six months since the policy was introduced in January 2023. The study also took in responses to a questionnaire sent to applicants.

The council’s policy goes beyond Part L of the Building Regs, requiring all new residential and major non-residential building developments to achieve net zero energy in operation, by cutting energy consumption and matching what energy is consumed with onsite renewables. Offsetting is allowed only in exceptional circumstances.

Furthermore, all major developments must keep embodied carbon, covering material production, transportation and construction, below a threshold value, with no offsetting permitted.

The targets are considerably stricter and more challenging to meet than Part L because absolute values must be met instead of relative reductions based on notional dwellings.

Most applicants said they expect the policy will be effective in reducing operational and embodied carbon emissions and lead to broader long-term environmental and cost savings. Most applications received by the council were for minor residential buildings, eligible only for the net-zero operational energy policy.

Researchers found that only around half of incoming applications appeared to comply with the new policies and blamed a lack of policy awareness for the errors. Many applications ‘lacked transparency, making proper scrutiny challenging’, the study states, which makes a case for more detailed information to be submitted.

All applications proposed grid electricity for heating and all but one included air and/or ground source heat pumps, which researchers said ‘indicates a general phase-out of natural gas in place of electrification and a focus on heat pumps for achieving net-zero compliant developments.’

Matching renewable energy generation to demand was perceived as the most challenging target to achieve, with many non-compliant applications failing this criterion.

Most compliant applications included the recommended U-values and air permeability values, although some respondents said there was a high risk of air tightness performance levels not being achieved in construction, which could lead to late-stage planning condition failures that are costly to remedy.

The policy targets air permeability of 3m³ /(m²hr) at 50 Pa and U-values of 0.16 W/(m²K) for external walls; 1.2 W/(m²K) for windows; and 0.12 W/(m²K) for roofs.

All applicants were concerned about the cost increases needed to achieve compliance, particularly those related to generating on site renewable energy and hiring external energy or sustainability consultants, which were expected to disproportionately affect smaller projects.

Will Hawkins, principal investigator at the University of Bath, said these concerns could be alleviated in part through the creation of a set of exemplary design templates, showing the various ways compliance can be achieved.

‘It seems many smaller design organisations are likely to look to external consultants to assist with the required modelling for the policy, which is an understandable cause of cost concern,’ said Hawkins, adding that the existing PassivHaus framework would be an effective means of ensuring compliance with the policy in terms of cutting operational emissions.

Asked to what extent Bath’s planning approach will be replicable in other local authorities, Hawkins said the university hopes to establish this through ongoing research. ‘There are a growing number of local authorities with very similar, or more ambitious, proposals in the pipeline,’ he said. ‘There will be some growing pains, as always with new regulation, but it will quickly become normal procedure. A key risk is a council’s ability to police the policies, since it takes time and expertise to properly scrutinise the feasibility of what’s being submitted. Streamlining this is another area we’d like to tackle.’

Researchers highlight the opportunity for further projects, at Bath & North East Somerset Council and beyond, to assess the impacts on real emissions as buildings are designed in detail, constructed, and occupied.