When we were selling our old house we had 2 possible completion dates one in December last year and one at the beginning of January this year. Our lawyer advised us to sell in December otherwise due to recent legislation we would be liable to pay IBI for the whole of 2016. Is this correct?

I didn't know it was recent legislation but it's what I had always assumed.

It's like car tax, whoever owns the asset on January 1st, is responsible for the charges in that year.

I am not so sure re car tax. Whenever I have sold a car and not replaced it, the tax was charged on a quarterly basis. Whether this differs for other areas of Spain I couldn´t say.

You can make a claim for car tax that you personally don't "use" - and yes, this is done in quarterly chunks.

... but it's not a given. If you don't claim it back, you have to pay the entire year.

When we bought our house we exchanged contracts etc on the 14 Jan. The seller kept putting the IBI bill through my door. I was confused and told the seller that I would enquire with the solicitor as I had paid him to ensure all outstanding bills etc were paid. The solicitor said that bill was not mine because we signed it over etc after 31 Dec which made it the sellers responsibility. Had we signed it over pre 31 December it would have been our bill. They did say you could agree to pay half each but legally 31 December is the magic date.

Surely when you agree on the price of the property it includes an adjustment for the IBI paid pro rata or about to fall due. It is then built in to the sale contract prior to signing. This is poor negotiating all round if not so.

I'm not sure I agree.

From the buyers perspective (in this case), if they know that it's law that the person owning the property on 1st Jan is responsible for paying it, then why should they negotiate.

Perhaps poor negotiating on the part of the seller though not to have spotted this.