True story: something very similar happened to me.
Rented a tiny first floor one-bedroom flat back in 2012. It was the first time I’d got my own place as opposed to house-sharing - the place was an absolute BOX but it was my own space anyways.
So I’d been in the place just over four months, and at the beginning of January 2013 it was time to take the Christmas decorations down. With storage space being practically non-existent, I asked my dad to fetch a stepladder around so that I could stick the decorations into the attic, which I’d never even been arsed to look in.
So my dad steps up first, turns on a light, and exclaims “There’s an entire room up here!” I thought he was on the wind-up, but it turns out there was one of those sets of attic steps installed. So I step up and lo-and-behold, there’s an incredibly ornately decorated attic room, with floor space that is bigger than the actual flat itself (it extended over the flat next door too).
There’s a desk, a futon, an office chair, CD player, a collection of CDs and cassettes (lots of metal), soft lighting installed into the wooden finish on the walls, an old map of the town framed on the wall, and a fucking connected LANDLINE which allows me to dial out - the contract can’t have ended yet. I hadn’t even bothered to plug a phone into the landline downstairs, as I was working in a call centre for minimum wage and couldn’t afford phone/internet.
There was also a selection of small axes and other woodworking tools, which made things even creepier. Anyways, a few months later I got into conversation with the guy who lived downstairs (used to call him Matrix Man because he was about 6ft 6in tall and wore a leather trenchcoat and shades all-year round). He asked me if I’d been in the attic.
Turned out the previous tenant was a joiner who had passed away, and the landlord was seemingly unaware that the attic had been secretly renovated - or if they knew, they didn’t advertise or mention it as it probably fell foul of regulations.
It was creepy as fuck though and I didn’t really spend much time up there - it just felt like I was invading a dead man’s space, especially with all his personal artefacts still in place, so I just left it closed up.