Only open to the public recently, this is still a hidden gem in the middle of the hustle and bustle of the Paseo de Gracia. Sometimes when I walk by, I look at all those hundreds of tourists who only have eyes for one particular building: The Casa Batlló. However, they do not look to the left, where right next to it is another beautiful building, the Casa Amatller, and a little further down the road, just at the corner, the Lleó i Morera House. Nobody looks at it because it happens to be in the same block as the amazing and famous Gaudi Building. When I take family or friends to see Barcelona, I make sure I point out the other buildings, feeling that I am sharing a privilege with them. The Lleó i Morera House was built by the architect Lluís Domènech i Montaner, the same architect who built the Palau de la Musica Catalana, the Hospital de Sant Pau and many more. I am a big fan of this architect, I admire Gaudi and his genius, but I feel that other great architects are not being paid tribute enough, such as Lluís Domènech i Montaner and Puig i Cadafalch.
The Lleó i Morera House can be visited, but you need to reserve in advance.
Find contact details here http://www.casalleomorera.com/en/visit-the-house/general-information/
Note that at the moment of writing this post (April 2015) the house is closed for visits on Mondays!
By the way, if you want to see the original furniture and some art which was made for this house, you can see it in the MNAC Barcelona (hopefully one day they will realize the importance of bringing the furniture back to where it belongs).