That’s an excellent question, but the answer depends heavily on how you define “longest to build.” There are several contenders, each with a different claim based on continuous construction, multiple phases with long interruptions, or a single, incredibly long building period.

Here are the most famous contenders, each holding a record in its own way:

1. The “Longest Continuous Construction”: Cologne Cathedral (Germany)

• Period: 632 years (1248 – 1880)
• Why it holds the record: This is often cited as the record-holder for the longest period of continuous, purposeful construction on a single architectural plan. Work began in 1248 in the Gothic style, halted for nearly 300 years in 1560 due to lack of funds and interest, and then was faithfully resumed in 1842 using the original medieval plans and finished in 1880.

2. The “Longest Overall Building Period”: Milan Cathedral (Italy)

• Period: 579 years (1386 – 1965)
• Why it contends: While construction was more intermittent than Cologne’s, the cathedral was in a state of ongoing development, modification, and addition for nearly six centuries. The main structure was largely complete by the early 1800s, but details, facades, and statues were worked on until the final gate was inaugurated in 1965.

3. The “Ancient & Ongoing Project”: Cathedral of Saint John the Divine (New York City, USA)

• Period: ~130+ years and counting (Ground broken 1892, still incomplete)
• Why