The policy of the Secretary of State is to be very selective about calling in planning applications and generally only does so only if planning issues of more than local importance are involved.
You can read more about this on DCLG website.
In addition, the Government will sometimes call in applications that the local authority has turned down and applicant has appealed against.
In effect this means that the decision is made by the Secretary of State in person rather than by an inspector appointed by him.
This is known as 'recovery' of a planning appeal.
In 2005, some 150 appeals out of 25,000 in total were recovered in this way.
If an application is called in or an appeal recovered, in most cases there will be a public inquiry.
The Secretary of State will, in general, only consider the use of his call-in powers if planning issues of more than local importance are involved. Such cases may include, for example, those which in his opinion:
However, each case will continue to be considered on its individual merits.