Hi Tim
If you rent for less than 30 days per year then you don’t have to apply for a holiday rental licence. But there is nothing to stay you can’t offer longer than 30 day stays with your H.U.T. registration (although in other regions this stipulation is in force).
If you are in Barcelona City and don’t yet have a licence, it’s unlikely you will get one at the moment, as there is a freeze on licences in the city.
However, if you offer stay of a four week plus duration, you can offer accommodation under the LAU (Ley Arrendamientos Urbano – Urban Tenancy Act). Unfortunately you wouldn’t be able to advertise your property as tourist accommodation. So no advertising on sites like HomeAway, Airbnb, etc.
Hope this helps
Louise