When you buy a residential property off plan from a developer you pay exactly the same taxes as you would were you to buy a finished property from a developer.
You will pay VAT of 10% on top of the purchase price (it was 4% if you completed before 01/01/2013). Any stage payments you make will include VAT.
When you sign the public deeds before a Notary you will also have to pay 1% of the property price (excluding VAT) in stamp duty.
Finally, as is the case when you buy any property in Spain, you will have to pay the notary fee and the land registry fee. These vary according to the price of the property and the number of clauses in the deeds. Combined they shouldn’t amount to more than 1% of the price of the property.