A typical clad wood window projects approximatley 1" to the exterior side of the sheathing. We often use a 5/4" flat casing and rabbitt the edge on three sides to overlap the window frame by approximatley 1/4". The wood siding then butts to the trim.

Remember to use a metal flashing above the windows and doors, and bevel the lower casing trim to weep water.

We haven't used Pella products for about 20 years, but I do not remember their products projecting as much as 1.5" beyond the sheathing. If this were the case, I'd install the wood siding and butt it to the window frame, and then lay a 3/4" casing trim over the siding. I'd forgo the rabbitted edge, and ensure that I have drip flashing excellently applied.

If you are using a stucco application or another thin material in lieu of wood siding, I'd suggest that you "build-out" the stucco at your openings, or ensure your foam backing on all walls will dimensionally compensate for the wide window and door projections.

The Pella Design Manual says the frame (including sill) on an aluminum-clad ProLine Series window projects 1 5/16" from the nailing fin. Andersen is the same. Marvin is about 1 1/16".

Unfortunately, the smaller the jamb projection the shorter the sill will be.

Thanks for the information. Would it be possible to use a 1" thick trim and run a bead of caulk to between the trim and the window frame or would this look bad?