Welcome to the forum.
Side decks could be arranged to fold out, making for reasonable foot paths fore and aft, while being able to fold up for over the road transport, within the max beam requirement. Lisa B. is a poor example of what she is. Very little is known about her deceased designer, whom didn't appear to be a professional, so asking questions about the design is moot, in this regard.
Connie, you have to make some decisions about what you want from this design, before actual pen to paper time is attempted. Budget, weight targets, speed targets, amount of underway time anticipated per year, sea state conditions you normally will expect, etc. What these decisions will do is refine the design, so you can approach how much hull volume you need, how this volume should be distributed, given it's static and underway needs and the practicality of the build. We call this an SOR (Statement Of Requirements).
For example, after a basic GA is worked out, you might find you need another 2 more feet in length. Easy enough right? Well once you work out the weights for this seemingly modest length change, you find the boat is now 60% heavier. How can this be you'll ask, but it's really simple physics, some of which you might not be aware of yet. My point is, even though it seems a simple thing, a floating box, depending on your needs, budget, skills and other requirements, it might be a wee bit more complex a set of issues to contend with, than first glances might suggest.
Free plans generally are what you pay for. I know Chuck at DuckWorks fairly well and he thinks good things about these (Lisa B) plans, but hasn't built one or poured over them looking for details either. My point is, Lisa B and boats like her, really shouldn't be asked to do much other then supply a place on the water, you can call home for a weekend. These types of boat motor poorly and can't handle much more, than a easy afternoon putter around the sheltered cove it's parked in. I have several designs that offer similar accommodations, yet can venture out into a some chop, stiff wind, take a beating and bring your butt home without fear something going to fall off, in route. This said, there's lots of reasonable cruising designs that will do the same, without having a Winnebago or old fishing shack thing, parked on a barge looking contrivance. Simply put, it depends on your SOR (needs) and these will define the boat, the way it's hull is shaped and built, how it'll handle underway, etc., etc., etc.
Can you list your SOR?
This is Glen-L's "Waterlodge" design. It's the size and general shape you want, though the math and the details have been worked out for you. If memory serves me, the plans are $100 bucks and you'll spend more than this just on paint for this puppy, so not a significant cost in the big picture, given what they provide.
This is one of my designs. The first thing you'll notice is it looks like a boat, not a old crab peeler's shack on a barge. It's trailerable, can take on rivers, bays and modest coastal cruising. A houseboat/shantyboat can't do any of these things, except get dragged on a trailer. Again, you get what you pay for . . .