No access? Build it.
This home was oddly designed. It had a 4-ft closet with bi-fold doors, a short 2-ft wall and then a 2-ft door. Behind the bi-fold doors, which never stayed on the tracks, was the HVAC system. It took up the full space.
Behind the 2nd door was a sump pump next to framing under the stairs. Between the HVAC, the framing, and the short wall was the water heater. There was absolutely no access to the water heater that if it broke, you could not change it!
We removed the bi-fold doors and installed a 6 panel sliding door. Then we removed the single door and cut the wall out. We added a header under the stairs and removed the framing. In addition, we added a coupler to the sump pump pipe so it could temporarily be removed if needed to change the water heater. Next, we framed the new 4-ft doorway and installed another set of 6 panel sliding doors.