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Generally, any building which requires planning permission is also required to comply with building regulations. In addition, even where planning permission is not required, certain works must always be approved by Building Control, such as adding new drains. You can use a small building as an office, workroom, gym or other incidental use, however, if you plan to use the building for sleeping accommodation, it will need both planning permission* and building regulations approval. Compliance with building regulations where applicable is a legal requirement. If you build foundations, connect to mains drainage or create any permanent structure designed for residential use, it will need to comply.
As long as any work carried out meets Building Regulations, approval cannot be withheld, however, if you fail to obtain approval, your project can be stopped and a building inspector can insist that work already done and covered up can be dismantled for inspection.;
See the UK Government planning portal for further guidance.
Building Regulations and Mobile Homes
Mobile homes, otherwise termed as park homes or lodges are not regarded as permanent “buildings” and as such are not required to comply with building regulations, with the exception of certain groundworks and connections to mains drainage.
They are however residential dwellings and will need planning permission* unless you are specifically granted an exception by your local council.