In Genesis's Noah's Ark story, the first structure Noah built after leaving the ark was an altar.
Genesis 8:20 says Noah built an altar to the Lord after leaving the ark. He then took some animals and birds described as clean and offered burnt offerings on it. The scene marks the transition from survival during the flood to worship after the waters recede.
The altar is significant because it appears before the later covenant passage in Genesis 9. It is also followed by the biblical statement that God would not again destroy every living creature in the same way, though the rainbow covenant is described afterward.
Noah's post-flood actions are sometimes summarized as beginning with planting a vineyard. Genesis does mention that later, but the text places the altar first. The answer therefore refers to the first construction recorded after Noah exits the ark.