prefix
... years since construction; ... years old (of a building)
Prefix used before a number of years to indicate the age of a building since its construction. Common in real-estate ads, e.g. 築10年 means '10 years old'.
築5年のマンションです。
It's a 5-year-old apartment building.
suffix
built in ...
Suffix used after a year or era name to indicate when a building was constructed. Often seen in property descriptions, e.g. 1980年築 means 'built in 1980'.
この家は1990年築です。
This house was built in 1990.