I don't know anything about planning / building, but does the planning office not inspect sites periodically to make sure the plans are being followed?
only one in 10 council planning departments are fully staffed, with 13% operating with fewer than three-quarters of posts filled
with
80% [of local authorities] reporting they did not have enough officers to carry out their workload
because
long-term cuts to funding have had a visible impact on planning departments’ ability to retain staff... there has been a 43% fall in resources to the planning system from local authorities since 2009-10.