Even if you know what you want to do, a good set of plans can bring to light otherwise unconsidered opportunities. If you intend to hire anyone for any part of the project, plans will help to ensure that all your bids are pricing apples to apples and that the person bidding has all the information he/she needs. It's disheartening and expensive to learn that the subcontractor and yourself are not on the same page. Also, even though you may know exactly what you want, a designer can expose design flaws that may turn off a future buyer or subtract from the value and at least bring them to your attention so you can make an informed decision. You could always just sketch something out and hire a designer for a consultation.

Scroll through the bathroom renovation sub-forum on houzz and you will see some nightmare installations - without proper waterproofing done by people that hold themselves out as contractors and/or some DIY installations. Getting the plans right with a professional will save you money, even if it sounds counter-intuitive. You don't know what you don't know.

Bathrooms are trickier than they appear. They look small and simple but they are not. Tile layout and finish pieces need to be determined, yes, weight of a cast iron tub with live load needs to be determined as to what kind of floor support-are you on a first floor, or second floor. Drawings allow all trades to understand not only bidding time and materials but locations. So, my answer is yes, get a professional on board. I am in full agreement with Elizabeth Minish Design, the cost of bringing a designer in can far outweigh the cost of mistakes you may make.

"My question is, if I already know how I want things to look and what will fit, etc. would I benefit from consulting an interior designer or architect?"

Yes. You'll benefit from having a professional set of eyes review your ideas.

What waterproofing system are you specifying? Does your handyman have the skill to not ruin that investment? How can you tell? What tile pattern have you drawn out for the shower? What decoupling membrane fit the floor? Do you know the required spacing for a toilet? A vanity? A shower? Do you understand cabinet construction techniques beyond the stupid buzz words if “plywood and soft close”? Do you understand the properties and tradeoffs if natural stone vs ceramic vs porcelain? Do you understand how to work with hard ducting got your bath fan? Do you understand about the needed CFM and location for a bath fan? What electrical and lighting is planned? Lighting over your tub? Moving plumbing is hella expensive and getting placement wrong for decirative piping for a freestanding tub is expensive twice. Do you know the advantages of a drop in tub vs a free standing one?

You beyond do not know what you don’t know.