Extraordinary fees

We are a community of 54 apartments built approx 2006 on 5 levels up a slope to the beach. Our monthly community fees are about 250/month with 2 none payers. Our President is an excellent hard working Spanish lady, and the administrator is also very good. We have just had our AGM and a vote was approved for an Extraordinary Fee of 9000 Euros (nine thousand) per apartment. A small number of members are refusing to pay this amount which is for earthworks at ground level to improve drainage and stop damp; 2 apartments are uninhabitable. My understanding is that we are all legally obliged to pay this amount. Are there any grounds for the refusers not to pay, or can another meeting reverse this vote?

A tricky situation.  I believe that the charge is legal and enforceable.  But equally the amount can be reversed or changed at a subsequent AGM/EGM.

Obviously I don't know the detail, but perhaps the work should be arranged in phases, if this is viable?  

With 54 apartments you'll always get some who are unable to raise that amount too quickly.