I'm thinking he's dead, or was just trolling.
But seriously, OP, either you've mis-measured and the wall/door is actually at the boundary of where your part of the building meets that of your neighbour, and through it is ... their attic... or your house is actually an SCP... or has arrived here from a horror movie. Or is just the scene of something unspeakable but currently benign, that will freak you the hell out anyway.
I'd say... get some friends together, but not everyone you know - you want some people to be on the outside, knowing what you're doing and to be expecting you to call back (like us, except, we don't know where you are!) so they can send in the cavalry if it all goes tits up - and a good length of rope, a powerful LED head-torch, 2 or 3 desk lamps and an extension cord powerstrip, a video camera, its AV out cable, and a looooong 3x coax extension (not plain) cable.
Inform the stay aways. Plug AV breakout into camera, plug breakout into AV extension, plus that into some kind of video recording device, with fresh media in, that feeds thru to a TV also. Hold camera in hand, LED torch on head. Tie rope around waist at one end, attic beam at the other, and have a rope-wrangler too. Take a desk lamp as well maybe, or at least use them to keep the general attic AND the target area brightly lit a the same time.
DOCUMENT THE WHOLE THING AND KEEP EVERYONE ON ALERT.
And... jeezus, please, don't blink!
...
yknow i think i SO need to try this in my mum's house now that she's on course to sell up and move quite soon. Just scatter a load of somewhat creepy, and by now quite out of date artefacts along one edge of the attic to make things look older than they are. Chuck a plastic skeleton in there too. Board it up just along that edge. Don't even put a door, just a dog-sized hatch. Hilarity ensues when they go up and try to work out why its been done on etc