Shead has, roundaboutly, put a finger on the issue the OP is having. Most stock plans for "farmhouses" (as generic and purposeless as that term is in terms of architectural styles) result from a designer drawing a pretty facade, and then playing room tetris to get all the required spaces into the outline they've drawn. Which is why most of them are dysfunctional. A genuine person of design talent designs both at once, so that as the interior spacial relationships develop, they also create a pleasing massing on the exterior.

OP, when you say "farmhouse," what are you looking for?

Greek Revival?

Federal:

Saltbox:

Victorian:

Italianate:

Gothic Revival:

Dutch Colonial:

Swedish:

Or that which I have dubbed "Suburbafarm Pastiche"?

In the case of all but the last, they tend to have layouts typical to their particular style. If you've walked into one Greek Revival farmhouse in Michigan, you can probably unerringly find the bathroom in any of them. That's because the interior relates to the exterior.

Now, a talented design professional can create a home that evokes any of these styles, while also arranging the interior to suit modern lifestyles.

But if you're going to have the luxury of designing and building something just for you, and you're not constrained by a suburban lot/HOA - say, actually building a farm house - then it's worth your while to spend some time looking at what farm houses were like. Because they had to, first and foremost, function. While at the same time, many were really lovely.