Legal Question in Business Law in Washington

Re: Altering, adding words to contract after signing

Generally speaking, once a contract is signed by both parties, that signed document is the total and complete agreement between the parties. The contract may be modified by the parties by a writing, or by conduct. But, if one party changes the agreement unilaterally by inserting additional or different language, then that would be done without the other party's consent and the change or changes would be ineffective. The only good way to prove that situation is to have a copy of the contract as originally signed to prove the unauthorized changes that were not agreed to.