We need an attic tool to create rooms inside roofs without having issues with the staircase

That would be a dream! I attempted an attic bedroom for the first time recently and I was so frustrated. Also, to be allowed more freedom when painting the walls attached to a roof would help.

You can use ladders. Sims will already go inside roofs as long as there's enough space.

Sims will use attic space as long as there is enough height clearance (similar to when building platforms or stairs).

There are 3 issues with accessible attics:

• You can't see the attic room(s) on the Gallery.
  
• You need to play walls down on that floor or else the roof is in the way.
  
• Interior roof walls have the same wall texture as the exterior roof.
  

I built a couple of starter homes with functional attic bedrooms/bathrooms, so you can see what I mean for testing purposes.

Gardener's Loft - Starter (you might have to swap out/move the coffee table to light the fireplace until you have the automatic upgrade, but otherwise playtested fine)
https://athena.thesims.com/v1/images/1/0x00001493/5F4A6583-DEA9-11E8-8296-6995527B6279/00.jpg


Skylight Starter #1
https://athena.thesims.com/v1/images/1/0x00001e3f/29A6A153-6916-11E9-8670-7E6A61392AEA/00.jpg