The enforcement period for outright planning permission is 4 years and for lack of building regulations 1 year. For change of use it is 10 years. Hence, this is out of time for enforcement by the local authority. What would normally happen is that he would be asked to provide an indemnity policy when he sells the property because that is the quickest cheapest and easiest way of dealing with it.
Alternatively, he can apply for a certificate of lawful use or retrospective planning permission. He may not get retrospective planning permission and the certificate of lawful use process is extremely convoluted.
Can I clarify anything else for you?
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If the neighbour structure has been there for more than 4 years without getting noticed, and hence, out of enforcement time, although there is no guarantee that if you applied for it, you would get it, the fact that the neighbour has this is going to be useful although not absolutely binding.
The council may not be too bothered about one of these structures but might not want a whole swathe of them.
The period for outright planning permission is 4 years so, if it doesn’t get noticed for 4 years, you are in the clear.
Just because it may be away from the house does not necessarily mean that it escapes planning permission because it depends on the exact extent of the work and if this involves raising the level of the land, the council will always look at it more closely.
It is indeed 300mm. This refers specifically to deacking.