We are remodeling a 1935 Home.
The current windows are as illustrated on the right.

Would like to ask your input about the planed window frame rebuild as illustrated on the left.

Does anyone see a problem in the planed new frame?

The illustration viewed from the top is exactly as its now (not to scale).

The 2x4 & 2x6 (yellow), and a new-construction window will be the only addition to what exists now.

Thank you.

 

I would consider replacement windows that do not have the outer nailing flange. You can fill the weight pocket with a 2x4 but leave off the 2x6 and the window gets screwed through the sides for mounting. That way you don't loose another 3" of window width.

 

Thank you for responding Pilot Dane.

There are other reasons that we will need the 2x6 maybe even 2x8. The wall thickness will increase as we are planing to add another 1” of rigid foam insulation, plus 3/4” of new plank board wall on the inside of the room.
My only concern, is if the trim (as shown) is the correct way to do it. ... Thank you.

 

If you don't want to make the windows smaller you can cut the interior sheeting/drywall/plaster off flush with the framing in the wall. Then add your 2x6 or 2x8 as shown in your sketch.

Outside is the biggest concern since you are not tucking the window frame or mounting strip underneath your siding. I never like to rely on just a bead of caulk to keep water out. At the minimum I would use flashing tape to seal the window flange to your exterior siding. Then apply the top trim with a healthy bit of caulk behind it. Then caulk the gap at the top. Better would be if you can get some metal flashing tucked up underneath the exterior siding to direct any water to the outside of the window mounting strip.

 

Not actually. If your goal is to keep your existing trim, you would be using some nice 1x lumber or edge banded 3/4 plywood or veneered jambs to make your jamb trim... not just a 2x6 or 2x8.

The casing usually has a 1/4" reveal where it meets the jamb. So the jamb goes behind the casing at least 1/2". There is some space in the rough opening... 1/4-1/2" per side that gets insulated... and that determines the size of the rough opening. And that's *if* you are basing everything off your existing casing.

If you are retrimming the windows inside and out, I would Sawzall everything back to your existing rough opening to make the windows as big as possible. Or just add a single 2x4 to the sides... the rest will be jamb and trim. But no, you shouldn't add the 2x6 or 2x8.