Hello,
Generally, as long as the house is untouched, the developers' warranty is in force, subject to the length of time given by the developer, which is usually one year after turnover. An additional construction/extension usually terminates the warranty because plumbing and electrical components are added, which the developer can no longer guarantee.
In my opinion, if anything was faulty, the new owners should have found fault with it when they moved in. You were liable for the repairs too at that time since you had your own modification already.
However they had their own construction as well, so who can say if their own contractor did some damage. Did they say what was the cause of the damage?
In order to hold you liable now, they must provide evidence that your contractor made some structural or other damage when the extension was made. Have their contractor (engineer preferred, not just any unlicensed person who knows how to make houses) provide a report and you can act on that.
In my opinion, your warranty ended when they commenced construction.
Should you be summoned to the baranggay for mediation, go. Ask them for proof of shoddy work by your contractor. If this is the first time you've heard of their complaints, say so.
It would be a different matter if the developer used substandard materials, but still it would have to be proved as well.
Good luck.