Posted: Tue Jun 29, 2021 3:42pm
As a Spanish resident, you have to declare and pay tax on all worldwide income. However, Spain has dual taxation agreements with the UK (and, I expect, other countries) meaning you can't be taxed twice.
However, my understanding is that you would pay tax in the country in which you work and receive payment (for example, UK), then you need to declare in Spain all the income received & tax paid.
Whilst the tax rates are similar in Spain, the personal allowances are only about 50% of those in the UK, so, in a basic example, if you earned £20K, with a £10k personal allowance in the UK, you would pay £2k tax (20% x £10K). £20k - £10k = £10K. £10k X 20% = £2k.
However, the personal allowance in Spain is £5k, then you would need to pay a further £1k in tax in Spain £20k - £5k = £15K. £15K x 20% = £3K. £2k already paid, means a further £1k due.
This is purely my understanding so you should really contact a Spanish tax professional to confirm. I'd be interested to know if I'm right!