Attic in the wall?

That sounds like an unfinished space to reach fixtures. Be care full of the insulation if it is fiberglass that stuff is bad on skin and if you breath in in not to good for the lungs either.

My old house had those. It was built in 1920 as a 2-up/2-down 2-family house. At some point in the 1950s a previous owner decided the kitchen in the upper flat was too small, so expanded it by removing a bedroom, then finished the attic with 2 bedrooms, 2 walk-in closets, 1 regular closet, & a little door on the stairs landing that was our attic, where my parents stored holiday decor. You had to use a stepstool to get at it & then dive in to get out what you wanted. It was at floor level with the bedrooms & hallway, so you couldn't stand up in it.

But there was a tiny little sliding door in the regular closet, & a piece of wallpapered wallboard that could be pried out in one of the walk-in closets. You could barely squeeze thru that tiny door. When I got older, I thought it was silly to have made it so little because that closet literally hung over the 1st flight of stairs & had a real wall. When I was little, I used to pretend The Borrowers lived back there. Nothing to see except pink insulation, dust, spiders, wires, pipes, & ductwork.

Same on opposite side with the secret panel. The 2 walk-ins were "separated" by the chimney, so you could go into one closet & exit from the other by going around the chimney. Is there a chimney hidden in your converted attic?

The person who turned the attic into extra bedrooms rather cleverly cut in boxes (to seal out the dust etc) at the end of each bedroom wall, & made 3-drawer dressers in both rooms. My mother used them to store bedding because there was no proper linen closet. There used to be one outside the bathroom, but because it was pretty big, they made it into a walk-in closet for the remaining downstairs bedroom.

Kneewalls are there because someone decided no one wanted to sleep in a triangle-shaped room, & more importantly, there needed to be room for all the stuff that was running thru the attic prior to conversion, so it became standard building practice to move over & wall off the mechanicals & put them behind that wall. It also meant you were able to put bedroom furniture up against the walls like in any other room, instead of having it all pulled toward the center to avoid the slope.

I'm going to assume a converted attic does have a real floor. Because the main purpose of doing this was to corral the house's mechanicals, the builders would assume that some day in the future this stuff will need to be serviced, & left the existing floors be. Or if they had to install a subfloor, it would've gone wall to wall, no matter where interior walls were framed in.

It's a real floor if it looks like dusty old hardwood planks, it's a subfloor if it looks like plywood & you can see screws attaching it to the joists. It's probably safe to walk on. You won't see absolutely nothing for flooring because those attic joists are what's holding up that converted floor of bedrooms, & they tie into the trusses that go up the slanted exterior wall (the roof, really) in there.

If you're going to use that space for storage, it's going to need a super-thorough vacuum 1st, & not with a regular household vacuum, as there's who knows how many years of dust & bugs in there. You'd kill a regular vacuum. You need to borrow one of those industrial strength vacuums used by builders. Vacuuming will be a pain, since you can't really stand up in there. But you don't want your stuff sitting in decades-old dirt etc, right?

If you wanted to, you could wallboard over the insulation, box in anything electrical, & it would be a great secret hideout for little kids, if you have any! Isn't in fun to discover unexpected surprises in a new house?

The area you described is built in order to create wall space in a room where the roof line goes very low on one side or the other. As not to waste the space it is created for storage with access through a small door. The term that is typically used for this type of wall is a knee wall because it is not the full typical Wall height to reach the ceiling.

Some people use the space for storage,whole others create built in book cases or install knee wall chests of drawers (custom made or prefabricated). Built-ins  make good use of the space while freeing up thefloorspace within the room.

If you go to Pinterest.com, and use the key words "kneewall storage ideas",  you can make good use of some of the space while still keeping a portion the way it currently exists.

I have several of those! One connects from the back of a built-in cupboard in a walk-in closet to another closet behind a fireplace in another room. It used to also have an opening in the maid's stairwell, which has been boarded up and refinished. Three others connect under the eaves (behind the knee walls) between multiple top-floor rooms, accessed via removable veneer panels. There are some electrical cables and heating pipes accessible through there.  Another one just gives access to the separate, unheated "attic" inside the conical turret.

sounds kyool post some pics

Sounds like it's a space under the roof, behind what could be called a knee wall.  That's a wall that is usually pretty short, three feet or less, to close off the relatively useless space between the lower parts of the roof and floor in an upstairs level.

You can use that space for storage but be aware that there may not be a proper floor there, so be careful to either first lay some kind of flooring down, or else don't put anything down on the ceiling of the room below - only span across the joists.

Part of the attic that resides behind a knee wall. Obviously some of the "attic" space is finished off.

https://en.wikipedia.org/wiki/Knee_wall

It can be called your attic if you like. The roofing style of the house is what determines where there is storage space.  At the peak there is very little that is usable so there is no access to the peak.  To me attic is above the head. Cavities in the wall are closets...or storage areas as the angle of the roof was part of the wall, so as most furniture is made to go against a vertical wall not a sloping one a false wall is built that is vertical. And you have empty unused space...between the vertical wall and the actual roof. That area could be just blocked off and not made use of, or in your older home, it was made use of as a storage area.  You can hide illegal immigrants in there...or other stuff like anything.

Chalet style houses usually have a "ghost walk" for the triangular area where the roof comes down to floor level. They're useful for getting electric cables through. You may find that you can get in from either end.

I used to live in an old house that had that.