Allowable Solutions are being proposed for the 2016 zero carbon homes target as a vital element of the policy that will assist the development of cost effective, high performing homes. Allowable solutions will also help the government and industry in unlocking bigger off site carbon offsetting opportunities.
The reason why not all carbon emissions can be mitigated on-site is because there may be technological or spatial restrictions that would make usual solutions either unviable or inefficient (compared to other options).
At this point (April 2014) it should be stressed that Allowable Solutions are a largely undeveloped concept. While broadly they are being envisaged as a diverse range of carbon saving projects, their selection criteria, verification methodology and practical application, in particular, are all subjects of ongoing debate. Further details are available in the Zero Carbon Policy section of the website which can be viewed here.