Absolutely no way we can tell what will work with the very limited info given, but you can be assured of one thing, your way will not work. The entire structure needs to be taken into account with the spans, lumber size and depth. You need a super experienced contractor or an engineer before doing anything.

If you are going to say that it will not work, please back it up with your reasoning. The foundation has been redone with more posts, piers, and beams. The floor joists have been tripled. the load bearing walls have at a minimum 2x4's studs spaced on 12" centers. Many places we added even more 2x4 studs. Why would my plan of sistering the attic 2x4 with 2x6 as well as stacking sistered 2x6 below the sistered 2x4/2x6, essentially making a 2x12.. not work? I will also add either a single 2x6 or sistered 2x6 in between the 24" centers to create 12" centers.

It's difficult to comment on a question like yours without all the details.

There are span tables you can use to determine joist size.

I'm not sure what you mean by "as stacking sistered 2x6 below". It sounds like you are just using a bunch of wood in hopes that it will be enough. Studs on 12" centers is an example.

And what about needing to create a dropped ceiling? Why is that?

https://www.jlconline.com/how-to/interiors/beefing-up-attic-joists-for-living-space_o

I am not arguing. The poster said there was not enough information. I am attempting to learn why it would not work. Just saying no it wont work doesnt help me learn anything. I have obviously never done an attic conversion before and am trying to gain as much knowledge as possible before starting.

I will take a shot at this... How are you planning to tie the upper and lower 2x6 together?

First sistering the attic 2x4 with 2x6. I need to create a drop ceiling below for electrical and drywall. If I stack sistered 2x6 below the sistered 2x4/2x6, essentially making a 2x12.

So how are you transferring the load of the upper 2x6 to the lower 2x6? I mean you mention sistering them and stacking them to essentially create a 2x12. Why not use a 2x12 then and avoid the extra labor needed to transfer the load from the upper 2x6 to the lower 2x6?

For this to work you will essentially have to remove the (shiplap?) ceiling and attach the upper and lower 2x6 together with metal plates or plywood and a nailing schedule. You would spend half as much money just hanging the 2x12 down on joist hangers, or use 2x10's or even 2x8's (which would probably be fine on a 12' span on 16" centers). There might be a way to make what you want to do work, but it is not likely to be worth the money or effort.

@bry911 This article describes it better than I can: https://www.jlconline.com/how-to/interiors/beefing-up-attic-joists-for-living-space_o

The houses original studs and attic joists are on 24" centers. I have added studs in between on 12" centers. I can't add 2x8 to the attics joists or I will not have the head clearance needed. I was thinking adding sistered 2x6 on the 24" centers with hangers (below the attics sistered 2x4/2x6) while also adding one 2x6 pocketed on the 12" centers below a newly added 2x6 in the attic.

I was thinking of cutting into the shiplap ceiling but not removing it completly (to keep all the 2x4 joists tied together, then the new attic subfloor will hold all the 2x6 joists together ) and attaching plywood to both upper and lower 2x6 (like shown in the article) to transfer the load from top to bottom. I also thought sistered 2x6 was stronger than one 2x8. Thoughts?

@bry911 Oh, i really like the idea of sistering it by aligning with the top of the 2x4.. Would you recommend notching the sistered 2x10 around the top plate of the load bearing wall so 4 inches of it is supported by the wall while the rest is supported by a hanger? Is it safe to suspend almost an entire second floor by relying on hangers?

I would think you need 4" above (so 4" notch) and 3.5" or 5.5" below unless you are going to use some sort of support plates. Which works well with your existing 2x4's.

The actual design should be approved by an engineer but most business facing lumber yards will have an engineer on staff who should be able to work out the attachment details for you at little or no marginal cost.

Lumber in a home that's over 110 years old has had ample opportunity to lose strength due to moisture, insect or fungal damage. It's best to have the existing condition assessed on site. Use of LVLs in lieu of dimensional lumber joists is an option to consider if ceiling heights are an issue.

Consider removing the wring and reinstalling it after properly sistering the joists. A lot will depend on the accessibility of outlet boxes but you won't need an engineer.