The laws governing planning applications in Spain can be complex with different levels of government and beurocracy, so it can take time to be granted the permissions you have applied for. Make sure you apply a few months in advance of when you are hoping to start work.
Using an indepedent and bi-lingual lawyer or planning consultant could take the headache out of the process.
Pools require regular maintenance and cleaning (twice a week in summer), so if your property overseas is a holiday home or you are going to be letting it out, you must employ someone to maintain it in your absence. You may be able to ask friends or neighbours to look after it in return for allowing them to use it.
Hot tubs, Jacuzzis, spas and whirlpools are also popular, but will also require ongoing maintenance.
If you have an outdoor pool, put a plank across it at night, so that any animals (or humans) that fall in have a chance to get out again!