Many thanks for your patience. Whilst ultimate responsibility lies with the owner of the property on which the repairs are being undertaken, there is joint responsibility with the person who carries out the building work. They will also be responsible for ensuring that the work is compliant with the relevant Building Regulations.
So as far as the law stands, enforcement action for breaches of the Regulations will be taken against the property owner as that is who Building Control can prosecute. However, at the same time the owner can consider taking separate action against you for negligence, if they can show that you were negligent in the process and that you should have obtained the necessary consent before starting the work.
I think the issue here is that you knew that there were going to be potential requirements for satisfying building regulations but you decided to proceed without checking these. You would be compared to a reasonable and competent builder in your position, one with the same knowledge and expertise as you and would be judged as to whether you should have known better and taken the necessary steps to ensure it was done as per the requirements. If you ignored these requirements and did so negligently then you could be made liable for the costs insured as a result of your negligence. Saying that, if the property owner knew that what they were doing was wrong and they also decided to proceed with that knowledge, they would likely have contributed to these costs and should be held partially liable. So the liability will be split in some respects. It means you may not necessarily get away with this scot-free but also not be responsible for the full costs of resolving this.
This is your basic legal position. I have more detailed advice for you in terms of the options that you have now and what could happen next, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there is no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you