I’m afraid every case may be different…
I can tell you in brief to clearly specify what are you purchasing, avoid vague descriptions, (add a complete house description, number of rooms, the actual square meters, the common services and infrastructure), check they have a building license and make them attach a copy of the license to the contract, specify the completion and building dates & warranties (don’t be fooled by the so-called 10 year building warranty), add retention clauses, add a penalty clause for building delays, specify that completion will be understand when they have the last habitability license… and, of course, sign no document and pay no Euro without an independent legal advice, don’t let them press you, take your time…
Look in the main page of this web site, there is an interesting and reliable guide to buying Spanish property off plan
http://www.spanishpropertyinsight.com/spanish_property_off_plan_property_spain.htm
Cesar