what does your solicitor advise?
not trying to be funny but if the building had structural issues and needed to be rebuilt then i would suggest the rebuild costs be knocked off the asking price. obviously the seller will reject this but if you think of worst case scenario that a structural warranty will give you then that's the cost you'd incur without one.
if it's a semi detached though if one side has a warranty then i can't understand why the other side wouldn't? if structural issues happen on one side then won't that also affect the other side?
tbh, i don't have a clue what to advise here apart from speak to your solicitor! maybe others will be along to offer something else.