Swimming ponds currently fall into the same official category as swimming pools, and according to current regulations (Town and Country Planning Act 2015, Part 1, Class E) you generally do not need planning permission if:
1. Your property is not listed, and
2. The site is within the curtilage of the property, and
3. You are not in an Area of Outstanding Natural Beauty (AONB)
The regulation stipulates other requirements but these three are nearly always the main ones. In addition you should be aware if permitted development rights have been withdrawn for any reason (usually at the time of previous development), or if there are any tree preservation orders (TPOs) on trees which are close to the pond or necessary access routes.
If your property is listed, or if you want to build on agricultural land, or if you are in an AONB, then planning permission will be required.
We have a lot of experience in getting planning permission for swimming ponds and still have a 100% success rate in more than 100 applications spanning 18 years, so please talk to us if you need to discuss further your specific situation.