Ministers should extend the deadline for local authorities to formulate local development plans, because more than half have still not completed them, leading conservation body the National Trust has demanded.

Councils which haven't adopted local plans - indicating where developments could take place by the end of the month - would be subject to the National Planning Policy Framework's ‘presumption in favour of sustainable development'.

However, nearly a year after publication of the NPPF, some 51% of English councils have still not updated and adopted local plans - a research conducted by the Trust and think tank the Local Government Information Unit has found.

Additionally, more than a quarter (26.8%) of England's local planning authorities estimate it would take a further calendar year for their plan to be adopted.  Just less than one-in-five (17.5%) expect plans to be completed in the next 6-12 months and 6.7% hope to finalise plans within six months of the end of March deadline expiring.

Peter Nixon, director of conservation at the National Trust said: ‘Councils need more time to get their local plans in place to protect land from unwanted development and ensure communities get the developments they need, in the right places. Only in this way can development be genuinely sustainable.'

Jonathan Carr-West, LGiU Chief Executive, said: ‘Planning is one of the most important but also one of the most contentious functions that local authorities perform. In a tough economic climate it's really important to balance the role of development in driving growth with local needs and aspirations. There may be real tensions between the two. Local area plans provide a way of working through these tensions but it is not easy.'

Malcolm Sharp, president of the Planning Officers Society, said a year's transition period was not long enough to complete the local plan process.

‘Planning authorities are being asked to do local plans, support neighbourhoods, put the community infrastructure levy in place and negotiate infrastructure delivery - it's a big ask on them to keep all the balls in the air.'