If they have a current valuation, from a priced round, they can probably pay; I'd go that route. If they don't have a priced round, I'd recommend code at a rate

If they have a current valuation, from a priced round, they can probably pay; I'd go that route. If they don't have a priced round, I'd recommend code at a rate in exchange for convertible debt, with a reasonable discount on their next priced round. It's just cleaner, though it could have an impact on their future financing, but that's not your problem. For example, let's say they agree to $150 per hour and a 20% discount. Then, you give them 1000 hours worth of work. you have $100000 worth of debt (technically, plus your interest), they raise a round of $2m on $4m and you are getting $100k worth on $3.2. Or, a scenario where they raise convertible debt for seed and the incoming investors see your discount for in-kind, not cash. That could be a problem. Here's your problem. Startups want to get as much as possible and given that you have a day job, that's going to create some friction. If they are hustling and pressed enough to go for this sort of arrangement, be prepared to be micro-managed, for free work. And what happens when they argue with you over how long something "should take" vs. how long something did take. Good luck.