Chimneys should extend at least 2 feet higher than any portion of a building within 10 feet, but not less than 3 feet above the highest point where the chimney passes through the roof. This is known a the 3-2-10 Rule for Chimneys.
Type L venting systems should terminate with a listed and labeled cap at least 2 feet above the roof and at least 2 feet above any portion of the building within 10 feet. This is known as the 2-2-10 Rule for Type L Vents.