If you are creating a bathroom in an area of the house that is currently used for another purpose then the use of that area is changing and you will need building regs. This is not a complicated process. Just contact your local Council and ask to speak to 'building control' talk to them about your project. There will be a fee to pay based on the contract price and they will tell you what they need to see. Typically they will be more interested in the drainage arrangements, the inclusion of an extractor fan and that any electrics in the area are compliant.