Being tax resident in Spain is not your choice
once you have made the initial decision to move to Spain.
Generally, once you have spent 183 days (not necessarily consecutive) in Spain, you are deemed to be tax resident and have to declare income and assets to the Spanish tax office. The tax year in Spain runs from 1st January to 31st December. Unlike the UK, which works on a part tax year basis when someone leaves the UK, in Spain you are either tax resident for the whole year or you are not.
As soon as you know that you will be taking the step to eventually become tax resident in Spain, it is extremely important to make certain that you have arranged your investments and property(ies) in a way that isn´t going to open you up to unnecessary Spanish taxes.
A lot of people will be looking to become resident in Spain before Brexit on 31st December 2020, in case the process becomes more complicated after. However, for those who are worried that applying for a residence card will automatically make them tax resident, let me dispel this fear. It does not. Therefore, you have the opportunity to apply for a residence card whilst taking action to protect your assets free from Spanish tax for 2020, becoming tax resident in Spain in 2021.