How does the cost of renovating an old house in southern spain compare with a similar project in this country?

The cost is very favorable depending on what you are doing .
BUT get permission before you start very important and in writing its a minefield different areas have very different regulations on what you can and cannot do.
Don't listen to anyone that say you can build this or that no one will notice Spain is air photographed every two years and a building on a plot is touched they soon pick up on this and if you have no permissions they can and do knock them down.
Don't buy so called Almacens(agricultural building) its a favorite build to do all the wrong things to get around the laws and illegal to live in .
get a good English speaking lawyer and take good advice from them before you do anything.
keep these in mind and use your head not your heart and you should be OK.disregard this and your build will cost you heartache,financial disaster,and me saying i told you so
good luck

It depends on what kind of renovation you have in mind. If you are talking about structural repairs, Spain is a lot cheaper than here. Building workers are easy to find, but you could have a language problem if you don't speak Spanish. You also need good legal advice if you are changing the property. Spanish law is very strange and very strictly enforced. Get a good lawyer involved if you are extending the property, or you may end up having to knock it all down again!