A quite a few years back we had a two story home (we don't have basements here in Oz) and I used the storage area under the stairs to built myself a small gun and loading room there.
This was before we needed safes to store firearms, I lined all the interior walls with 1" marine ply (made the area a lot more secure) I hung hooks from the back of the stairs for my rifles.
I ran a double neon light on the onterior wall and a bench below the light and also a couple of double power points. I had some 1/8 inch steel plate so I screwed this to the exterior side of the solid door (yeah, paint it the same as the walls) and used two deadlocks to secure the door.
You could still use this idea and just put your safe down the end and keep that in there too. I found this a good idea as guests did not ask what was this room for.