sql - using a case statement inside a string -

i need put case statement in string. how can achieved?

select datediff(mm, select case fa.new_ownertype         when 8 fa.new_vlenrollmentstartdate         else fa.new_contractstartdate bingmapsplatform_stagging.dbo.filteredaccount fa         end, getdate()) 

select datediff(mm,     case fa.new_ownertype     when 8 fa.new_vlenrollmentstartdate     else fa.new_contractstartdate     end, getdate()) bingmapsplatform_stagging.dbo.filteredaccount fa