Yes they do!
You have a leak. Whatever the cause and however long it takes or however expensive it is the housebuilder has a responsibility under the warranty to fix this defect.
As you have a ground floor flat it is highly likely the leak is from the balcony above. Failing this it is a plumbing leak from the flat above.
No home should let in the rain, especially a brand new home!
Who is the housebuilder?