What is the limit for late fees, for freelancers, in Washington state?

I would say that you could reasonably charge a maximum of 18% if the contract work is not for home, family and residential consumer purposes. I advise my clients that "net 10 days" is reasonable, and many businesses prefer, "all amounts invoiced are immediately due upon receipt. Interest at 18% annually will begin to accrue on any invoice not paid by the 10th day." Of course, 12.0% is defensible entirely, as the Washington State statutorily approved amount (at least for now), which is the rate I would utilize if the contract is of a type that typical consumers & family require. (E.G., if you go into homes to help set up computer, internet, and other technical work for basic family use). If your clients are businesses, go for the 18%. if you want to research this on your own, the term you need to look for is "usery." You can read more at: http://www.leg.wa.gov, and while their search engine is terrible, it will eventually get you there. Possibly http://www.mrsc.org would get you there faster, or maybe even just google "Washington law on usery." Good luck! However, as your business grows I would strongly advise you to have an attorney review your document to give you the maximum benefits of Washington law. There are many things a good transactional attorney can provide you, and good contract fences make great contracting partners.