Office space in San Francisco is ~$5 per square foot per month, and an employee needs somewhere between 100-250 square feet of office space. So if you give them a small 175 square feet, then you're at ten grand per year.

Medicare is %2.9 percent of salary, half paid by the employee and half by the employer. Social security is %12.4 percent, again split between employer/employee for 6.2 percent from each. . It looks like California's unemployment tax is another %4.2

I have no idea what the average cost per employee things like health insurance and other benefits cost; I'm sure you can google as expertly as I can, so now that I've shown some good faith in giving you a starting point I trust you're willing to take it to the finish line.

There's probably some other taxes that I'm missing, since I don't run a business and have never had call to learn about the ins and outs of these things. Usually it comes up in conversations around negotiating for raises to give somebody some perspective about how much they care about an extra ten grand a year (a lot) versus how much their employer cares (it's basically a rounding error compared to how much their bottom line for retaining an employee is).