The Parliamentary Under-Secretary of State for Communities and Local Government (Stephen Williams)

But making all new homes zero carbon “on site” is not always physically feasible or cost-effective for house builders, so the Government will also introduce a system to enable remaining carbon emissions abatement to be met through flexible, cost-effective allowable solutions. Allowable solutions will be “off site” measures to be supported by house builders. These could include creating or expanding connections to sustainable energy sources—for example, district heating schemes—or improving existing homes’ energy efficiency by retro-fitting measures such as solid wall insulation.

[Source]