The best option is undoubtedly the youth hostel, which is cheap, clean and only 5 minutes' walk from the Faculty of Physics. For more information:

https://www.reaj.com/info-albergue.asp?id=78

One possible option is the Colegio Mayor Sant Raimon de Penyafort:

The contact details are available on the following website:

https://www.penyafort.ub.es/?mod=colegis&pag=contacte

There are rooms for visitors, although it is not particularly cheap (room and breakfast for €52.10). A discount is applied for stays of more than one week.

Another alternative sent in by a student from a previous course:

In Barcelona I stayed in Residencia Nikbor; it was better than the hostel in Madrid but worse than the university hall of residence. For students arriving in Barcelona by plane, there is a local train from the terminal which leaves you right on the street where the hostel is located after four or five stops. There is also a Metro stop next to hostel which takes you to the Faculty of Physics, so it has a good location. Breakfast is included in the price and you can also order dinner for an extra fee by arrangement although it is somewhat expensive and is nothing special. I recommend that you go to the restaurants around Plaza Cataluña for dinner.

https://www.nikbor.com/

Accommodation in Madrid recommended by students from previous years:

• During the Nuclear Structure course I stayed at Residencia Erasmo, which is directly opposite the Faculty of Sciences of the Autonomous University of Madrid. It supposedly offers long-term stays for students, although the possibility also exists of short-term stays. It's relatively expensive but the room (with kitchen) is very good and the fact you have a kitchen means you can save on dinner costs (you can also save on lunches but normally members of the Master's course have lunch together). It's perfect for the Nuclear Structure course at the university because it means you don't have to get up early in the morning. You need to catch a local train to go to the centre but the station is relatively close to the Faculty. It's ideal due to its convenience and if you sort things out in advance the rooms can be occupied on a shared basis, which lowers the price.

                                                   https://www.resa.es/Residencias/Erasmo

• For the Applied II course I stayed at Hostal Madrid, right next to Puerta del Sol. All I can say is that it does the job for a bed and a bathroom; the room is small and there's no kitchen, but if all you want to do is sleep that's fine because the rest of the time you spend at CIEMAT. As it's near Puerta del Sol you can take the Metro to go to the Complutense (from where you get off to the Faculty it's around 15 minutes' walk, 10 minutes to the Faculty and 15 minutes to reach CIEMAT) and it's near many places to eat out.

                                       https://www.hostal-madrid.info/es/estaticas/index.asp?wm=56&id=23

•  A very convenient place to stay for the Experimental Techniques in Nuclear Physics course is Olé International Hostel in Madrid. This is a cheap, comfy and safe youth hostel for people who like to have a good time, make new friends and learn about different cultures and other ways of thinking as they discover Madrid. It's located in the city centre with rooms in the Malasaña district, which is famous for its busy nightlife. The young staff will ensure you have an unforgettable stay in one of the best hostels in Madrid. If you're looking for a cheap place to stay in Madrid... Online reservation for youth hostels in Madrid. Address: Manuela Malasaña 23 1º - 28004 - Madrid / Tel/Fax.: (0034) 91 446 51 65

                 HTTP://WWW.OLEHOSTEL.COM/MADRID/LOCALIZACION2.HTM