How does Scott Lang afford to live in a townhouse in San Francisco?

He lives in Apartment 2A, on the third floor of the building (which also includes the upstairs attic, making it two floors). You can see this when he's standing at his front door, which opens directly into a stairway, and the apartment number is on the door when the FBI burst in.

He also shares the place with Luis, who's wearing PJs, and is surprised to see the giant ant taking Scott's place. Luis is also there later, talking to Scott on the phone, and also says he hates the way Scott loads the dishwasher, so they're splitting the rent.

Furthermore, it's almost a year between the events of Ant-Man and Civil War, and then two years on house arrest after that, so there were almost three years in which to start and grow the X-Con security firm.

And while the business was on the brink of bankruptcy, it had a very well equipped office space, and had at least four employees, so we can infer that there was at least enough cash flow to cover their rent (for the time being).

He also repaired his relationship with his ex-wife (and her new husband), so she may have been helping out financially.