Johnny purchases a house, doesn't want furniture left behind. He signs Tuesday, changes mind Thursday, calls agent to add furniture to contract. What is this?

An option is a separate contract that gives one party the right — but not the obligation — to purchase property within a specified time period for a set price; it has nothing to do with adding terms about personal property like furniture to an existing purchase agreement.

A novation is the substitution of a new contract, party, or obligation for an existing one, effectively extinguishing the original agreement; adding a furniture clause to an existing contract does not extinguish or replace the original contract, so novation is clearly inapplicable here.

An amendment modifies or changes terms that already exist within an executed contract — for example, changing the closing date or purchase price — whereas Johnny is adding an entirely new term (furniture inclusion) that was never part of the original agreement, making addendum the more precise legal instrument.