Hi - I was very keen on building a log cabin for a while and looked into it carefully. In my area, which is not too far from Elche, a log cabin is dealt with in the same way as a permanent house and you need planning permission. You will also need to check that you are buying enough land to build on as, if it is rural land, you may need as much as 25,000m2 to build anything at all, and then you need to restrict the size of the building, including pool etc, to 2% of the land area.
Each town hall makes it´s own rules and it will also vary from plot to plot. Take the exact details of your plot and a description of what you want to do with the land and go see the town hall. Ask if there is anything you need to do, not just planning permission. I had friends who asked if they needed planning permission to glaze in their terrace and the town hall said no. So they did it and were then fined because they did not ask if they needed a licence, which they did need!
And Mr Kevin, there are also many decent estate agents who would not want their clients to have any problems with planning. Quite apart from the moral issue, we want our clients to recommend us in the future!