If a building is converted into a dwelling (a house or a flat) and occupied continuously for a period of 4 years, it becomes immune from planning enforcement action.
It is possible to apply to your local council for a lawful development certificate (also known as a certificate of lawfulness) to confirm that the development is now lawful.
In this case, our client had been using the outbuilding as a rented home for many years. We helped him compile and organise detailed evidence of the use and submitted an application his behalf.
We were delighted when the council confirmed that the use was lawful and issued the certificate.