Every Tax Authority aims to make sure that everyone is paying their taxes whether native or from aboard. This is no different in Spain. In fact, in recent years the Spanish governing bodies are trying to tighten up on their taxation methods. Spain is one of the largest economies in Europe, but in the past they have terrible trouble with people not declaring their income or assets, as well as many people in Spain being used to dealing to cash only.
The other problem that many countries with in Europe have had is tax avoidance all together. Due to an agreement between certain countries, Spain included, someone cannot be taxed twice for the same thing, like a pension for example, but one must declare where the taxes are being paid. What many people did was to avoid both countries altogether, which of course lead to a lot of people losing their assets to repay taxes, as not paying taxes when you are meant is a criminal offence.
The tax fines will continue to go up, the longer you leave it. At first you have a set amount of time in which to make the payment, if it is not made then the next payment period will be shorter and there will be interests as well. This will of course continue to escalate until paid. Un-paid taxes in Spain can result in your bank account being frozen in order for them to withdraw the fund directly. If it has been going on for many years the amount of tax that is due could possible exceed the value of your assets, which would consequentially means you would lose them.
Depending on the amount of time elapsed the tax will increase with interest, and then in addition with fines.
At the point of selling a property the taxes will be deducted from the profit made. When purchasing there must be a tax fund set up in the bank to cover all costs, which it is important that you consult your financial representation to authorize the correct taxes and amount to be paid. Otherwise the Spanish tax authorities will at some point reclaim the missing amount with charges.
Most taxes in Spain can be split into smaller payments to make things more manageable, but it is a good idea to not get to behind or avoid them. Seeking tax advice is strongly advisable for just as much nonresidents as for residents, because each will have corresponding taxes when you have a property.