I want to add an attic to an already built 2 story home, without disrupting the lower levels or most of the roof. My architecture skills are pretty low level. I was wondering if anyone more skillful would take a look at the place and advise me on how to do it in a visually pleasing way.
I took the pictures seen here with my phone camera, my apologies, it's the easiest way for me to access the internet while I'm playing. I'll upload screenshots for anyone who is interested.

I'm not keen on making houses, but from what I understand, there is no way you can add an attic without modifying the roof of that house according to that last screenshot. An attic typically is another level, which requires you to draw a wall and room then you do the roofing. I suppose it is possible when the roof is high-pitched. But in such cases, you still need to draw out a room else you will have routing issues.

By the look of the shots, seems you need to turn on "restrictbuildbuyinbuildings off" so you can attempt to place a floor in the attic and then try to see if you can draw walls. actually I can't really tell from the screenshot. Anyways, try adding a flooring to the attic then you can see. I notice you already placed a stair, can your sim walk up there? I doubt it b/c I think there'll be a routing issue with low ceiling.

I can be wrong, but that's from my understanding in the few buildings that I did play around with the roof.

I'm not sure how big of an attic you are looking to build but nitromon is probably right that you will probably need to be able to place attic walls and have room for a stair landing. Is stair stacking possible with sims 4 without issues? I'm also not sure what the lower floor layout looks like or if there is room to move the current stairs back a space so the landing comes out near same level as upper floor wall and move plant and put door to blue rug bedroom closer to window.More pictures that show doorways and floor below would be helpful. This might also be better in building to get better input.

I have put attics in many houses, and as said, there needs to be room to make one that can be used. The area has to be high enough and wide enough to place walls to enclose an area. I have always used stairs but a ladder may work to access. Odds are you will have to alter the existing roof to do something. Also, I tend to add dormers and often a very long one along the back of the house to make enough space for the area to be able to be used for really anything.

Okay so, to try and answer questions.
The stairs in the picture are the stairs between the first and second floor, they don't go up to the third floor (the attic). I plan on using a ladder to imitate the old hatch and ladder type of attic.
I've added flooring under the roof. I can draw walls, but only a 2 wide x extremely long section that is pretty unusable.
I intend to use the attic just for storage and looks, not really a place for sims to do anything, although it would be nice if they could get up there.
I don't want to change MOST of the roof. But the center gables, front and back, I wouldn't mind changing. I'm not really great with roof designs though tbh. I was thinking of taking the center and back gable off, and turning the back gable into a 3rd floor but it just doesn't look right. Even if using a high gable on it as well, it only has 2 wall sections.

I was wanting to add dormers, I'm really not sure where to put them. I experimented a bit but it all looked messed up to me.

If my build I would lose the front middle gable, and all in back. Make the extension in back a flat top that can be used and run a large dormer across the back in that area where you can have access to that area. It will give you much more indoor and outdoor space.

I like that idea, but I'm having trouble visualizing where you're suggesting the dormer go.

The issue is probably having at all room at the top. I would try to make the staircase fit on the second floor somewhere you can make the room work in the attic. Then you have to have at least 2 tile wide rooms in there, and cover the rest with roofs.

I would just rebuild though, but then I'm building 2 houses a day when I'm bored, so how do I know.

You could of course cfe your way to an attic, but I have a feeling you don't have the skills.

Yeah I don't like to spend hours on building at a time. I start with a really small structure and build on to it as needed. This house started as a one story single bedroom home that was quite cute, and I added the second story when the second gen was born and my sims had enough money to build and furnish in one go without cheats.
I'm definitely not interested in rebuilding the whole house, it's been in the family for three generations and I've put a lot of time in to decorating it, and don't want to spend the time rebuilding.
I know how to use cfe but I don't enjoy using it and would rather find other methods to expand.
I already said btw that I'll be using a ladder to mimic the pull down attic hatch look, so finding room for it isn't much of a problem.

Here is a house I did with an attic that can be used. In your case, you have an addition on the back. I would use that with a door access from the attic area. And I sure cannot draw well in Paint. LOL

This is what I ended up with, and I'm mostly satisfied (except that for some reason now the front gables have that weird glitch/gap? It wasn't like that originally and I'm not sure how to fix it, but I haven't tried removing the entire roof yet to fix it.)
The front gables are a bit bigger than I wanted, they looked at lot better at 2 width, except that I couldn't find a window I liked in that size, so I ended up making them 3 width, and still couldn't find a window I really liked. :'D I might eventually look for a cc window to go there.
While I really liked the look of turning the extension into a balcony, it wasn't quite what I was going for. I wanted the old "ladder hatch attic" look (especially since I didn't want to have to remodel the 2nd floor to fit a staircase in) and having a balcony up there made the attic into more of a 3rd story than an actual attic. Sometime in the future I might decide to expand it further, but I don't actually need the room right now (it's just two sisters living there since their parents died, and my main one is about to move to a new neighborhood via traveler, and the other one is a plantsim who I don't intend to have reproduce, so the house will probably sit vacant for a few generations until I decide to move back to sunset valley.)
There's enough room in the attic for all the junk I wanted to use to clutter it up, and room if I want to use it as a spare bedroom or something. So it'll be good for a while now. I moved it to a larger lot so there's room for the yard to be extended, I might add in a garage and I definitely want to add in a shed with a cellar at some point in the future. But for now, I'm satisfied with it.
Thanks for the help and advice, although I didn't go with all of it, it was helpful in determining the final outcome, and eventually if I decide to make it more of a 3 story home, I will definitely be added on the balcony, I wish I had gotten a picture of it when I had it set up that way, because it looked quite nice, and would be a lovely little place to put the tea table and look out over the yard.