The role of checking that building regulations are being complied with falls to building control bodies (BCBs).
There are two types of BCB, a local authority building control service and a private sector approved inspector building control service.
Customers are free to choose which type they use on their project and there are voluntary performance standards in place for BCBs.
If you are employing a builder to do your work you should be clear from the outset whether they are taking responsibility for ensuring that the building work complies with the building regulations, and also whether they are taking responsibility for liaising with the building control service you have decided to use. If the builder is registered with a recognised competent persons scheme, they may be able to self-certify some or all the work they are carrying out.
An additional reassurance will be to use an installer and/or products that have been certified by a third party organisation as being competent and fit for purpose. These 3rd party certification schemes are not normally an alternative to formal building regulations approval.
In addition, you should bear in mind that your building work may require planning permission. You will need to check the position and if you do require permission you will need to be clear whether you, your builder or designer are taking responsibility for obtaining it.
Some types of building development may also be subject to other types of statutory requirements and consents.