We stayed 3 nights here, with 4 people. Some things to note about Belinda's flat, some mentioned by other before:
The place is in a great location - walkable from the Madrid bus station (20 mins) and also from the Atocha train station (15 mins). It is also very close to several supermarkets, including a Lidl just a block away. The unit is located at the 4th floor, without lifts. However for the space itself, it is very cramped for 4 people, probably more suited for 2 comfortably. The place also feels quite cluttered with a lot more furniture than shown in the photos especially in the bedroom. The bathroom does seem to be newly renovated compared to the rest of the house - and the variable shower positions, water pressure and heater is great. There is a big water heating tank in the bathroom, but unfortunately for 4 people, it is not sufficient - sometimes in the middle of a shower the hot water will just run out and it suddenly becomes freezing cold (Usually the 3rd or 4th person showering back to back). This meant that after the first day we would stagger showers with around 45 mins to an hour between the first two person and the other two. When cooking, we continuously experienced the electricity tripping on us - so it might be some sort of overvoltage or something. The furniture and appliances except the bathroom is a bit old but everything works fine - but things like the stove's dial's markings have totally faded off so you have no idea on what heat level you are turning it to and it's all a guess. Finally we were a bit surprised because on one of the days when we were out, we definitely turned off the heater in the living room, and when we came back, it was already on so we suspected someone entered the unit and turned it on. We had valuables in our bags and felt a bit unsafe and uncertain about it and trying to communicate with Belinda about that - she did not reply to that matter despite being asked several times. All in all it was a decent stay, and Belinda tried to accomodate and be helpful as much as she could. Would definitely recommend for the location and if it's for 2 people, but again, we think it is quite cramped for 4 people.