This is a condition attached to a planning permission grant.
A planning condition limits the extent to which planning permission can be utilised at a property. Many local councils have either removed or restricted homeowners' rights to develop their properties.
Buyers are strongly encouraged to research the planning history of their prospective property to determine whether any special conditions could infringe on any developments they have in mind.
Simply ignoring or pretending to be unaware of these conditions does not protect the owner from prosecution.