Anyone knows whether there is a thing as comprehensive property insurance? Ideally, it includes fire/flood, and very less exclusion like earthquake/structure problems.

usually you insure your house and land value by using fire policy. additional like riot, malicious damage (RMD) are rider that you pay extra premium. you can add those riders but it will be impractical and very expensive

I suppose that structural problems could be added as riders then?

not too sure about that as new building is usually covered by contractor and developers insurance. After warranty lapse structural problem still goes back to the engineer who approved the plan and sue them for professional negligence. By structural we mean design defect not cosmetic defect like cracks etc. I think insurance company still need to fill the form and the riders offered is on the form. Get one and see

Just thinking out loud.... if I sign up for the comprehensive insurance that covers structures damages

..does it take effect on structural defects that happen BEFORE, or only for those defects that happen AFTER signing up?