Although it's a bit small, the Museum of Finnish architecture takes you through the last century of architectural design of the country. There's a small temporary exhibit on the second floor. Altogether the museum shouldn't take you more than 30 min. Free entry with the Helsinki Pass. Located behind the Design Museum, can combine both in a visit to this part of town, the Design District.
It's a small museum that exhibits History of Finnish architecture. Top floor has a good chart that shows history and a few examples.