There are new regs in place governing summer house rentals. As far as I can tell:-

1. You need a Cedula
2. You need an Energy Performance Certificate
3. You need a licence from the adjuntament
4. You need to check you get registered on Gencat with your house details, email & phone and max occupancy.
5. You need to register the names, pasports etc of everyone staying in your place on the Gencat site.
6. You need to collect the tourist tax (45c per person per night)
7. You need to pay your income taxes on all receipts.

Here are some interesting links:-
Spreadsheets of registered properties,
http://www20.gencat.cat/portal/site/empresaiocupacio/menuitem.32aac87fcae8e050a6740d63b0c0e1a0/?vgnextoid=658d7e7951d10310VgnVCM2000009b0c1e0aRCRD&vgnextchannel=658d7e7951d10310VgnVCM2000009b0c1e0aRCRD&vgnextfmt=default

Tourist Tax,
http://www20.gencat.cat/docs/empresaiocupacio/20%20-%20Turisme/Documents/Arxius/dipticimpostCAST.pdf

Background,
http://www.spain-holiday.com/rentalbuzz/holiday-rental-licences-in-catalonia

I suspect that, if the authorities chase this then the number of rented properties will collapse and the local economy with it.