You probably don't need to mention waterproofing except perhaps to say "seal joint between window sill and sub-sill with paintable silicone (or urethane, etc.) sealant.

The sub-sill should extend beyond the window frame at each side far enough for the vertical jamb trim to stop against it.

The siding should be installed under the sill-nosing so that the drip groove in the nosing projects beyond the surface of the siding. Sealant at the siding to sill-nosing joint might be appropriate too.

Flashing would be in the flashing spec or window installation instructions. Paint would be in the painting spec.

Advanced TrimWright, Inc. makes two sill profiles: standardSILL and historicSILL. Older double-hung windows had very large projecting sills which is why many of them are still functioning today and will probably be around long after a lot of modern nail-fin windows are replaced. Modern windows are intentionally designed so that the sill only projects as far as the jambs of the frame. This is easier to fabricate and also easier to put in a box for shipping.

Modern window manufacturers think little about waterproofing for residential and commercial buildings, relying instead on modern sealants that must be replaced many times during the life of the window. Your goal should be to avoid that costly maintenance chore by letting water drip off of the window clear of the siding below like in the old days.

Thanks macv!

Modern windows are intentionally designed so that the sill only projects as far as the jambs of the frame. This is easier to fabricate and also easier to put in a box for shipping.

Oh wow, I did not realize this part about the exterior sill being so different compared to the older windows. When I looked at the models at the Marvin showroom, I studied the interior, but hadn't really thought about the exterior from this functional point of view.

Your goal should be to avoid that costly maintenance chore by letting water drip off of the window clear of the siding below like in the old days.

Absolutely, I hear you! :-)

Thanks again!

Long ago when window companies started using nail-fins with field applied trim my brother-in-law asked me to look at a problem on a large housing complex on the ocean where he was the property manager. Water was appearing inside below the windows at the baseboard. Andersen reps had been out but their recommendations failed to remedy the problem.

When I looked at it I realized that water could run over the sill and back to the siding because the "sill" did not slope or project much and there was no drip. The only thing stopping water from going behind the siding was a bead of sealant which is difficult to apply effectively, can't be easily inspected, and has a short lifespan near the ocean.

The builder added sub-sills and the water stopped. Since then Andersen added a better sill to their 400 series double-hungs but the rest of their product line is still the same. Now I add a sub-sill regardless of the window design because I never know when someone will change the window manufacturer/model and it prevents the installation of "picture frame" trim which only increases the likelihood of water penetration and looks ridiculous.