I am taking keys soon to an apt. in Torrevieja and intend to partly rent it short term, via airbnb for example. Suppose I let it for 2 weeks in Jan, that means my tax is due 20th April at the latest? If I never lat it out after this for the rest of 2019 do I still need to do a quarterly tax by 20th July, 20th Sept 20th ? Or do I just fill one quarter tax form in (the first one) and then do a simple no-income tax form for the rest of the year/remaining 9 months?