Warranty on extension and loft conversion.

[CEvert]

When you are having a house extension or a loft conversion, is there a standard warranty that comes with it provided by the builder?
How long should the warranty be and are the builders legally obliged to correct faults if found later?
What happens if the builder disputes if a fault actually exists?
Or if they are no longer trading, who corrects the fault then?

If the builder has chosen a NHBC home warranty, is this premium built into the cost that he is charging or is that something that I have to fork out myself?
Obviously NHBC needs to inspect the work, so would employing a ?NHBC? standard builder be significantly more than a good builder? (is this extra cost worth it?)

I am new to this so excuse me if the questions seem trivial.