John Allen apparently came up with the idea of "Givens and Druthers". A bit of an odd term, but it's a sound concept.
"Givens" are just that. Things that are "given", i.e. cannot be changed. No use planning a 20 x 40 layout if your available space is 10 x 14. They can sometimes be open to negotiation, as you mentioned above that possibly you won't use the entire space. But they're a good starting point.
"Druthers", as is in "I'd ruther (rather), are things that you desire for the layout. These are more flexible and tend to be modified as the design changes. You start with the basics. For example, I love steam, so for me a roundhouse is a very desirable feature. Modeling BNSF in the modern era? Then a roundhouse would be colossal waste of space.
I would suggest that you consider both of these elements when designing. You don't have to get super fancy, though you can find suggested "Givens and druthers" forms on the web. But at least get some general ideas. What era? What location? If not a specific geographic location, at least the general idea. Rocky Mountains, Florida, something in between? Nothing wrong with a "generic" locale, but at least have some idea of what kind of elements you'd like. The same goes for operations. Continuous run? Switching? Passenger trains? City setting, rural, or country.
Start big, toss it all out there. Of course, you typically won't get it all (For example, no matter how hard I try, a 4 track PRR mainline, city, mountains, mines, engine terminal, passenger station, freight yard and industrial area for switching isn't going to fit into my garage unless maybe I switch to Z scale. But put them all down for starters. Then go through and pick the ones you want most, while also picking ones that are practical. In my case, a 4 track main would make my layout look more like a slotcar track, so that's out. But maybe I can do a (small) station, and have a place for an engine terminal.
Oh, and one thing I can't emphasize enough... Allow for staging tracks. Yards aren't supposed to be parking lots for trains. Having off layout staging so you can store some additional trains is valuable on nearly any layout.