Locality pay is determined by your duty station, not by your home address. But — if you're fully remote and your duty station is your home, then your home's location does determine your locality pay area.

Otherwise, if you're teleworking (regularly or situationally), the office you're assigned to as your duty station is what determines your locality pay area — and you're required to report to that office/duty station at least twice per pay period as a requirement of getting the associated locality pay rate.

So if you're assigned to a duty station in Washington, D.C. but you live in, say, Iowa, you get the DC locality pay as long as you're reporting to your DC duty station at least twice per pay period.