Alternatively ....
I went from a fast track course last year, straight into houses and have never looked back - perhaps that's because they all burned down after my soldering?
I used to be in office type jobs and must admit it was a very steep learning curve. However, most of my jobs now come from recommendations from the plumbing jobs I've done so far.
I ensure I'm reasonably confident that I can do the job before I take it on. I'll suggest they get someone else if I'm uncomfortable. In my early days I called out a heating engineer to get the boiler going again but now know how to sort those out. I keep learning new things all the time.
I feel the attitude with customers (friendliness, tidiness, full and open honesty and cleanliness) are more important than your qualifications. Your skills are probably better than the customer because they wouldn't be asking for help. Money is well down the list. Customers primarily want someone who looks and is trustworthy enough to be in their house.
And to answer the initial question, you don't need any qualifications to work in peoples' houses but you do need to make sure you fit things according to the regulations.
