It should normally be free. Parking a car does have a fee on most days except Sunday. Occasionally there are also concerts or events that have entry fees.
It is free to visit the Cathédrale. The museum has a fee but it is closed right niw.
The Cathedral is currently closed for tours, due to ongoing covid restrictions, but is open on Sunday mornings for the 11am service and Sunday afternoon for 4pm Evensong. The grounds are always open and free, and though there is a fee to park in the underground parking garage (most days except Sundays), there is street parking available, as well.
Free
The only time I went inside of the Cathedral was during an art exhibit called, “Les Colombes”, and I believe admission cost 10-20$. Normally, I don’t think it costs anything though! Good luck with your school project and definitely make sure to check out this place, if you find yourself in DC!
I visited the cathedral three days ago and it costs $15 for adults, $10 for students and seniors. Not free.
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