I'm a minister, and in my previous church we had a good sized car park which people did sometimes park in without asking permission. It was annoying, because although much of the time the car park was unused, or only used by a few cars, at other times the car park was full to overflowing, e.g., if there was a large funeral or wedding, or other event on in the building. It wasn't a massive problem, but we much preferred it if people asked first. That way, we knew who the car belonged to, why and how long it was there for, and we'd suggest that they park in the corner furthest from the church.
I currently work in a church outreach centre, which is used 6 or 7 days per week, daytime and evening. Because we're in a town centre where it costs to park on the street or in public car parks, we regularly get people who have nothing to do with us, parking in our small car park. The car park might be empty when they arrive, but we have lots of people coming and going at different times, many of whom have mobility problems and need to be able to park close by. Twice, recently, a car was parked so close to the end of the wheelchair ramp that it would have been impossible for a wheelchair user to get in or out of the building. We didn't know who the cars belonged to so couldn't get them to move. Sometimes people park their cars, block other cars in, then go shopping! We've had volunteers unable to get out of our car park when they left, and had have to be given lifts or take taxis to get children from school or get to hospital appointments.
Grrrr....
Rant over.
Just ask. If it's do-able and not going to get in anyone's way, they may well say it's fine.