Bed and Breakfast

welcome to the forum.
The short answer is yes you need a licence. You will need also to have a certificate on how to handle food.
The process, but only in Catalan
The law.
A list of classifications.
You will have to pay a tourist tax. Information here
You have to consult a gestoría on how you run the business.

Thank you so much for responding. This is very helpful.

So , even if it's just two rooms in my own house, it's not a business and it's via Airbnb, I still need a licence?

But it is a business and, quite rightly, there are all sorts of controls to make sure that your guests are safe which is why you need licences and accountants and everything else.

Forgive me. When I said it wasn't a business, I meant that we wouldn't run it as a business.
Any income would be declared and taxed as regular income. We wouldn't employ anyone. And we wouldn't offset expenses against turnover.

I do not wish to get into semantics about what is a business or not. Maybe you should take that up with the Spanish tax man or the gestoria. In the information i have given you will not state how you should structure the enterprise plan as that is for the previous mentioned.
I am sorry I cannot answer all of your questions. I have given you a big help to start looking for more information. I am willing to help more. That what forums are about.
I simply searched for it. I am fortunate enough to speak and write several languages. But not Catalan    What I do not understand is how this sort of enterprise is any different to the UK . You start one within the rules and regulations. if there are not any fine you may win. If there some regs then maybe you  feel you loose. 
If you had carefully read the links then I can assure you you will need to licence the property plus you have to have that food handlers certificate. One thing I forgot was you need also to have public indemnity insurance.
Maybe if you have a mortgage you may need the lenders permission too.
Spain is not the UK so one advantage here is you do not have to apply for planning permission for change of use.
ABnB has a reputation that is not very good.

Thank you so much for this. It's all very helpful.

Oddly, there are no regulations in the UK regarding B and B. Unless, you turn it into a business which is registered and audited. Then it is more regulated.
Airbnb has an excellent reputation in the UK and is completely unregulated. Indeed I've offered a room in my house, via Airbnb for over a year.
No licence, no registration, no rules, no regulation, no extra insurance.

Clearly Spain is different. However, I just looked at Airbnb for May on the Costa Brava. Over 600 places came up. Houses, apartments, rooms etc.

I wonder if they are all licenced? Somehow I doubt it.

Anyway, thank you so much for your input. I will now consult my lawyers.

All I can do is ask you to read the regulations in the links, Then read the listings you have found, plus read these.
The LocalOnline newspaper.
The Telegraph Newspaper.
[url=The Telegraph]El Pais[/url] in English.
The question is do you want to run something illegal? What happens if you neighbour reports you?
What happens if there is an indecent and the Guardia Civil are called?
What happens when you do not register for your taxes?
A guest of yours gets food poising and has to Hospital, what happens then?
Do you want to spend all of your time having to look over your shoulder?
I any of the above occurs then you will certainly see just how Spain is different.
Good luck

I have absolutely no intention at all of doing anything illegal.

Which is precisely why I'm asking the question.

Andrew M P Byrne wrote:

Forgive me. When I said it wasn't a business, I meant that we wouldn't run it as a business.
Any income would be declared and taxed as regular income. We wouldn't employ anyone. And we wouldn't offset expenses against turnover.

It would seem that The UK tax office is now looking into those who list on Ebay, ABnB etc because earnings have not been declared correctly
The Telegraph
from the article:

It noted other countries including France, Estonia and Belgium have already taken steps to tax income from these emerging companies in a different manner to conventional income.

They missed off Spain from that list.

RibeiraSacra wrote:

Andrew M P Byrne wrote:

Forgive me. When I said it wasn't a business, I meant that we wouldn't run it as a business.
Any income would be declared and taxed as regular income. We wouldn't employ anyone. And we wouldn't offset expenses against turnover.

It would seem that The UK tax office is now looking into those who list on Ebay, ABnB etc because earnings have not been declared correctly
The Telegraph
from the article:

It noted other countries including France, Estonia and Belgium have already taken steps to tax income from these emerging companies in a different manner to conventional income.

They missed off Spain from that list.

Most of the world is cracking down on Airbnb now because millions are being avoided in tax payments and quite too.

I have just recently found this article.
All Online holiday rental platforms are obliged to give your income to the Spanish tax man. News
Sorry about raising this old thread but this is an important and will affect the OP and other members thoughts about renting out without registering.

@Andrew M P Byrne

Before committing to a bed and breakfast venture in Catalunya, it's essential to research local regulations. Licensing, registration, criteria, inspections, costs, and tax implications vary, requiring thorough understanding and compliance.

@Andrew M P Byrne If you are from UK. Start at the Consulate, to see if you can open a business.