Andoni's flat is actually much more spacious than described, and perfect for a group of up to 6 people. The bedrooms are all towards the quiet back courtyard, there is a small balcony for smokers, beds and pillows are good quality.
Andoni answers all messages immediately, and gave us lots of good tips. The appartment is also closer to the center than it looks on the map. I can highly recommend it for a stay in lovely Bilbao.