Can you geofence a competitor's location?
Yes. Conquest geofencing is legal and common, but it needs the right offer to work.
Yes, and it is a standard tactic called conquest geofencing. There is nothing legally exotic about advertising to people who visited a competitor. You are not accessing their systems or their customer list. You are buying ad impressions against devices that were observed in a public commercial location.
Drawing the fence is the whole job
Most conquest campaigns fail on geometry, not strategy. A competitor dealership sits in a retail corridor. Trace the building and the customer lot only. If your polygon includes the shared access road, the restaurant next door, or the apartments behind the lot, your audience fills up with people who never considered your competitor.
Exclude the employee parking area if you can identify it. Otherwise you will spend a month advertising to the sales staff, who dwell there forty hours a week and will never convert.
The offer decides everything
This is where honesty helps more than tactics. Someone standing in a competitor's showroom has already invested time in that visit. A banner saying we are also here does not move them. What moves them is a specific reason to make one more stop: a price comparison they can check, an inventory item the competitor does not carry, a trade valuation, a service the other location does not offer.
For a powersports or automotive dealer, that usually means specific units in stock. For a medical practice, it usually means appointment availability this week. Generic brand creative in a conquest fence is money set on fire.
Set expectations before you start
Conquest audiences are small and cold. Run the retargeting window long enough to matter, typically 30 days, and judge the campaign on incremental visits and leads rather than click through rate. Geofencing at LevelUp requires $5,000 per month in ad spend, which usually supports a handful of well chosen fences rather than every competitor in the metro.
Related: What is geofencing advertising and how does it work?, Can you geofence a trade show or event?, Do you work with automotive dealers?, automotive marketing.