We stayed there for 9 days last month and it was very good, comfortable and quiet apartment and right by the river and the Vaci shopping street. The owner was very helpfull even before we arrived in his emails and gave us a lot of good advices about where to go and what to see and so on. Even when we got there he took us on a short walk to show us where everything is in the area so really good service. We are going again later in this year when the weather gets warmer so that we can enjoy the balcony and views. We really recomend this place to stay in Budapest.
Found this place on tripadvisor but they have a web site www.budapestvacationservice.com. The street name is Regi Posta Street.
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