Yes some (not all) interior designers could work with you on designing a moulding package for the home.

If you are hiring a builder, often the builder will have staff to help with these choices. When we did our spec book to put plans out to bid, we went though the trim options and chose ourselves, keeping cost in mind. Our builder had a book with the most popular trim choices. We could have chosen anything but fancier trims cost more so we went simple.

It's often difficult to select interior trim for a home even with a rendering and dimensional guides for assistance. Our practice is to include our clients' "best guess" for trim in our plans and pricing. Once the drywall is in place we install trim "mock ups." These include casing part of a door and window and installing some 1 ft to 2 ft sections of crown mouldings and base so the client can view it in the actual space. Changing moulding dimensions--within the practical limits of stock moulding shapes/sizes--isn't typically a big deal.

Thank you @anj_p. Yes they have the trim selection books, but it’s more about any extra trim and where to put it. Going up the stairwell walls and casing openings etc. as well as some columns I want to add trim to.

Design assistance should begin a lot earlier than now. Way too many homes are designed these days where there isn’t room for any trim or window treatments. And not all of them are amateur produced. Plenty of them come from production mill factories. Then there’s the modern vaulted ceiling that just doesn’t physically work with crown molding. A designer can’t fix issues that are designed into the architecture. Make sure that the architecture supports your want list. Changes on paper are far cheaper than in wood framing and drywall.

Talk to your builder and see if he offers any trim packages for stairways, dining rooms, family rooms, etc. The sky is the limit what you can do today with trim, like this dining room as an example with shadowboxes, chair rail, custom ceiling, etc.

@User it was one of the things I discussed in detail with my house plan designer to make sure we have room for 4” trim around every door and window. We are doing 10’ ceilings so lots of room up there. It sometimes feels like every question on here turns into what should have happened or negative in one way or the other. Even if I had never thought of trim before now, which I had, what I should have done is not beneficial at this point. I have my beams for my family room and coffered ceilings on the plan . I’m talking about picking out which trim exactly works with the style I’m looking for and deciding what walls to put accent molding on etc. not whether I should now add a cathedral ceiling.

If your home drafter has that level of ability, then they are the ones that should be advising you regarding the trim for the home. An architect would. Because they have the historical education and experience that qualifies them to create the cohesive design whole. Details like that would be discussed, and included as sections on the plans. No need to bring in last minute fluffers.

A few hours of time with a designer will be well spent. Make sure you have the current plans ( how often I’m given plans that don’t follow what was actually built) as well as your inspiration images.

@GN Builders L.L.C thank you! I definitely will discuss with him. I guess I’m just looking for guidance to end up with the look I’m after as well as where to put the focus more than what’s available to do and wondered if an interior designer is the person that can work with me on that!

@Charles Ross Homes thank you this is very helpful information! I’ll ask my builder if he wI’ll work with us this way on it!!