The US codes always make us Brits smile. Here, we have electrical standards - these are entirely about electricity safety. We have building standards and these deal with safety too, but from a structural pespective and then a smoke/fumes perspective.
I'm not sure what's really different in that regard....the US has building codes, that mostly come into play for new construction where permits are required.
I mean...if you want to gut your two small bedrooms to make one big room...you can do it on your own with out the permits or inspections, though if you ask, they will tell you that you "should" get the permit and the inspections so they can make sure you're following minimum codes.
Of course just about every homeowner at one time or another does some house remodeling without every doing any of the above...and even here, you can rewire your electric, run new lines if needed...and all without a certified electrician, because who will stop you...?...no one.
That said...when you go to sell you house, it's the same thing here...when they inspect, if they find something was added without proper permits, or if they see oddball electric wiring...you will not be able to sell your house until that is fixed or you cut the buyer some agreement deal, where they assume the burden and you lower the price...etc...
...BUT...that's a minor concern. The real BIG concern is if you house burns down or falls down or whatever...and you call you insurance, and they send an inspector who finds out the addition you did had no permit...or the wiring you put in was at fault for the fire...you're SOL.
They are not going to pay out.
So...you guys across the pond can smile...but then, so can we if we covered out asses and got the permits and paid for the certified electrician...etc...
That's not to say that no one does stuff without those things...and there are many simple things that you can do and forgo the permit...but the important thing is to know your limitations...and when it's something you SHOULD do according to codes, so that your ass is covered.
My recent studio build...I could have gone a more "laxed" route...maybe saved some $$$...but considering the magnitude of the project...hiring a licensed contractor, who was fully insured, and getting an architectural drawing, and all the permits and inspections...it was all about covering my ass.
My construction was/is 100% approved, up to code, and officially in the books. There is no liability on my end for that.
I've seen where people have done some small builds without permits...and when the town found out...they made them either fix it, up to code w/permits...or take it down.