An employee also owns the copyright to his/her code (at least in most European countries), the company often just gains the exclusive right to use that code through the contract with the employee (or the law, in some countries). So whether or not it is you intellectual property does not matter that much, what is important is what's actually written in your freelancer contract. If you build a custom software system for a company I'd assume they will not simply let you sell "your" exact code to their competitor afterwards, at least if it's something that has some inherent competitive value (e.g. not simply an off-the-shelf solution like a CRM or website). Most companies make sure this can't happen even when working with a freelancer, so have a close look at your contract (if you have one). If there's no explicit contract it'll depend on the country you're in.