Thinking about having a new bathroom fitted - do we need town hall building licence

You will need an Obra from the town hall.  It’s just about needed for everything you do and if a builder is doing the work they usually arrange it.  I would never try to get away without following the rules and it isn’t expensive.

Whatever you do within your existing home and without affecting main structures of the building nobody will know if you don't tell, i.e. re-tiling, re-flooring, exchange windows.etc. In any case insist in a writen estimate and do not accept upfront nand/or part payments, you will get trapped. 

Yes, you need one. 

Don't trust anyone else to do it, if it goes wrong it will be your responsibility so do it right from the start. 

A builder usually puts in the application but it is normally yourself who goes to the town hall to pay.  We have just done that on a new driveway. You get the documentation so there should be no cause for concern.  We have had 5 obra licences.

I think they will know if the police drive past and see there’s work going on 🤔🙄

No, I disagree. 

I always apply for the licence, I fill in the application so I know what I'm applying for and what I am granted.  That way there is no misunderstanding. If you want to put it in someone else's hands then obviously it's your choice. 

Certainly where I am there's a building inspector who will check if a licence has been applied for if he sees or hears any sign of building work, as will the local Police. Additionally I can't dispose of more than two sacks of rubble at the tip without the licence, or him checking against my ID and car details. 

Depends on how lucky you feel! 

Not sure what you are disagreeing with here.  We both seem to agree that getting an obra is the thing to do.  I go to the town hall, I pay the money, I have the obra licence.

Can the application be made online?

Not in my experience, you have to go to the town hall, but I live in the countryside in a very small village.