Getting an unauthorised occupant out
The route depends entirely on how the occupant came in. A trespasser with no claim at all is removed by a suit for recovery of possession. A former tenant holding over is dealt with under the tenancy framework. A relative or co-owner in occupation is a partition question, not a trespass one.
Self-help (changing locks, cutting utilities, physically removing belongings) is the mistake we most often have to undo. It converts a straightforward civil claim into a criminal complaint against the owner.
Unauthorised construction and deviation
Construction without approval, or in deviation from the sanctioned plan, is actionable both by the planning authority and by an affected neighbour whose light, air, access or structural safety is compromised.
If you are the one accused
Not every deviation is unauthorised, and not every occupier is a trespasser. Regularisation schemes have applied to certain classes of construction, long possession can create rights, and an oral licence or family arrangement may explain an occupation that looks like trespass on paper.
If you have received a notice, do not ignore it and do not continue building. Both make the eventual position significantly worse.