Not at all sure this will apply to self builders. Section 144 which imposes new building warranties starts with, "this section applies where a person (the developer) carries out a development in England that results in the creation of one or more dwellings"
Paragraph 6 then effectively defines "carries out a development" as construction of a building with a view to "granting of or disposing of relevant interests in one or more dwellings" i.e selling the dwelling on to a purchaser.
It appears therefore that if you are not building the dwelling with a view to selling it to a third party then there is no statutory obligation to obtain a warranty. I presume that will not prevent you selliing it on in the future once you have lived in it. How long the gap would have to be will only be tested should a case get to court. I would have thought the secretary of state would be unlikely to pursue an individual in the first place and then only if they could "beyond reasonable doubt" show you had no reasonable excuse to sell on without a warranty. I'd say if you've lived in it a year, hate the area or divorce the partner and decide to sell up the SofS isn't going to come after you!