coeficiente

This is puzzeling me  how is the coeficiente calculated on an apartment and who would do the calculating?

It’s the percentage of the constructed area, of the whole, which you own.    Simple put if you have 5% of the area you have a coefficient of 5%.   All the coefficients add up to 100% .   It is included on the escritura (maybe shown as CUOTA) and is determined when the plans are drawn.   That is why it is almost impossible to add to ones ‘constructed area’ as the percentage of all the owners would need to be changed. Say if one wanted to close off a passageway which only leads to your front door, so as to include it in your property.

Usually when there is a vote at the AGM the coefficients are added to see if the overall percentage has been obtained to pass the motion.

Some Communities just do a simple ‘one dwelling one vote’.  It depends on what the Statutes (rules) of the community show.

John's answer basically covers it - I presume the developer calculates the percentages, and they are included on the master deed for the entire property when registered at the land registry.

On a side note, I purchased an off-plan property some years ago, and noticed that the amount the administrator asked me to pay represented slightly more than my percentage of the total budget. On querying this, the admin told me he had discovered that the total cuotas came to 99.42% only, so the annual budget had to be adjusted slightly, and my resulting payment was slightly more accordingly! So, just for the sake of contradicting John, All the coefficients do not always add up to 100% !!! 

Presumably the descrepancy on the deeds came about from rounding up the coefficients. It could only happen in Spain!