The building regulations are there to protect you and your family and you're ultimately responsible for making sure that they're complied with.
This reflects the fact that you'll have to pay if work has been done badly, both in terms of the costs of fixing the problem and in relation to the health and safety of everyone living in your home.
If building work doesn't comply with the building regulations, local authority building control can prosecute and fine you or the contractor doing the construction work.
Bear in mind that any faulty work that needs to be fixed will have to be paid out of your pocket.
You're also responsible for any planning permission required, and this is why you should always speak to your local authority building control department (enter your postcode in the search above for your building control team's contact details) and your local planning department before you start work.