Only a few days have passed since I am based here in Porto, and I already found out which the oldest house here is!
The house is named “Casa do Beco dos Redemoinhos” and it’s situated very near the city cathedral (to be precise, just behind it). It is dated in the first half of 14th century. Of course, not all authentic elements and details are preserved to the present day. It belonged to a rich merchant who traded with northern European countries. Therefore, there are some Gothic elements (Gothic arches at the doors and windows, two original windows preserved out of 4) and influences of Flemish architecture visible on its façade (the chimney is placed in the middle of the façade, just like in Flanders of that epoch).
Unfortunately, the gate towards the house was locked yesterday at the time of my visit, yet I found nice photos on the web for the blog. And a nice short film about the house, though in Portuguese: http://www.jn.pt/live/Programas/default.aspx?content_id=3650556
For this discovery and the link, I am thankful to my special guide through Porto.