In the 2011 budget, the government confirmed that from 2016 all new homes would be required to be ‘zero carbon’.

In England, the definition of a zero carbon home was one where CO2 emissions from regulated energy use (space heating, hot water, lighting and ventilation) were limited and mitigated by a combination of three measures:

Achieving minimum Fabric Energy Efficiency Standards (FEES) based on space heating and cooling.
Using low and zero carbon technologies and connected heat networks to limit on-site built emissions.
Where it is not possible to reduce the regulated CO2 emissions to zero using these on-site measures, the remaining carbon emissions can be mitigated through off-site allowable solutions.

Unregulated energy (such as appliances and cooking) was not considered within this definition. So in effect, developers would be required to avoid or mitigate all regulated emissions using a combination of on-site energy efficiency measures (such as insulation and low energy heating systems), on-site zero-carbon technologies (such as solar panels) and then using off-site allowable solutions to deal with any remaining emissions.
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However, on 10 July 2015, the government published ‘Fixing the foundations: creating a more prosperous nation’ a government plan for increasing Britain’s productivity.

Amongst a great number of wide-ranging changes, the report states, 'The government does not intend to proceed with the zero carbon Allowable Solutions carbon offsetting scheme, or the proposed 2016 increase in on-site energy efficiency standards, but will keep energy efficiency standards under review, recognising that existing measures to increase energy efficiency of new buildings should be allowed time to become established.'

http://www.designingbuildings.co.uk/wiki/Allowable_solutions_for_zero_carbon_buildings