I am self-employed, but I wonder, is it necessary to register as autonomo, or could I just declare my income and pay tax on it? I work as a freelancer and get my jobs online, my customers are mostly in UK and Germany, none is in Spain at all.

Also, I would need to register for social insurance/healthcare, but do I need to be registered as self-employed to do that in the first place?

Hmmmm.

You can't 'register' for social security/healthcare unless you are working. To be working, you either need to have a contract of employment with a company or be self-employed.

It doesn't matter where your customers are - if you work here you need to be registered.

That's a really interesting question mand I'm afraid I don't have the answer. There are very few 'experts' here and my advice is that you get the help of an expert either a really experienced gestor or you could ask the UK DWP who do post on here from time to time. It may well be to your advantyage to stay fiscally in the UK or even in no mans land. You will have to pay tax here of course. But would a comprehensive health insurance cost more than the Seguridad Social (What we call the Autonomo) payment? If you want to live retire and die here it's worth paying Autonomo if you can. If you are just here for a while I'd continue to pay in the UK. If I were to go back and do it again with the benefit of hindsight I would have looked at doing it that way. By the way have you considered the very high postal costs here? As soon as you move you need to contact the post offices sales and try to negotiate a 'deal' with the parcels Dept. My husband has only just done this after 5 years here and it does make a significant difference. You need to be able to predict your usage as they will ask loads of questions about that in order to tailor their offer to you. EDIT Sorry I didn't realise you don't do a lot of posting. Left that bit in as it might be interesting to expats who do online sales.

you should register.

the only way you'll be able to access healthcare here is if you register as self employed / autónomo here - & if you're living here & working here you need to do that anyway - it doesn't matter where your clients are, as snikpoh said

have a read of this Moving to Spain - what to do & when