Most building work in the UK will require some level of permission before it can go ahead. This is required for many types of building work in the UK, including new builds, extensions, and changes of use, in order to ensure that the proposed development is in line with local and national planning policies. 

One of the main reasons why planning permission is required is to ensure that new developments do not negatively impact the local community. For example, a proposed development may be denied planning permission if it would cause excessive noise or traffic, or if it would block the sunlight or views of neighboring properties. 

It is always best to obtain planning permission before starting any building work in the UK, as failing to do so can result in enforcement action being taken against the property owner. This can include fines, orders to halt work, or even the demolition of the work that has been carried out. Additionally, if you are planning to sell the property in the future, a lack of planning permission can make it difficult to sell the property and may reduce its value.