Dear Carol:
I would say it is the responsability of developers (or technical direction? ( plans) , or builders? ( execution of plans) as the building has been planned and it is being built under the effect of the accesibility legal requirements:
REAL DECRETO 556/ 1989, de 9 de mayo, por el que se arbitra medidas mínimas sobre accesibilidad e los edificios. (B.O.E. º 22 de 23 de mayo de 1989). This is a legal minimum which can have been implemented by a Comunidad Autónoma regulation. SO the specific legislation for your case depends o where your building is located in Spain.
I need to say too, that those responsabilities are not easy to determine by a Lawyer. I know the Law and can check the specific details of that construction but… the final responsability is determined by a Judge or an arbitrator.
If the building is still under construction I would encourage you to write a certified letter to the developers by which you ask for the information regarding the suitable conditions of the building under the regulation I have mentioned above. Best regards,
Maria de Castro