The Local Planning Authority (LPA) has a specific time in which to determine a planning application (8 weeks for minor applications, 13 weeks for major applications and 16 weeks for applications requiring Environmental Impact Assessments). If no decision has been made within this time period (except where the applicant and authority have agreed a longer period in writing), the applicant can appeal to the Secretary of State on the ground of non-determination. The procedures for this are similar to those which apply when an application has been refused. The Secretary of State appoints an Inspector from the Planning Inspectorate to consider the application.
If possible, it is often best to avoid appealing against non determination as the process can take a long time. The Planning Portal provides more information.
An Inspector when assessing this type of appeal will essentially be determining the application. Appeal forms and information booklets explaining how to appeal are available from the appeals section of the Planning Portal website.