When I saw upper floor open to the 1st, I was reminded of a passive solar plan... So you're gathering ideas... I bet people will be on soon.

https://sustainability.williams.edu/green-building-basics/passive-solar-design/

I can't contribute about living off the grid. Of course, I could be wrong, but I assume some aspects of passive design would help. I think hot day air rises easily in your area open to the 2nd floor - that's all really....

"The oldest and simplest form of convective cooling is designed to bring in cool night air from the outside and push out hot interior air. If there are prevailing nightime breezes, then HIGH vent or open on the leeward side (the side away from the wind) will let the hot air near the ceiling escape. LOW vents on the opposite side (the side towards the wind) will let cool night air sweep in to replace the hot air.

At sites where there aren’t prevailing breezes, it’s still possible to use convective cooling by creating thermal chimneys, designed around the fact that warm air rises...."

"Thermal mass absorbs heat and moderates internal temperature increases on hot days. During the night, thermal mass can be cooled using ventilation, allowing it to be ready the next day to absorb heat again. It is possible to use the same thermal mass for cooling during the hot season and heating during the cold season."

Where do you live?

"The climate determines the best natural ventilation strategy." ... what direction is the prevailing breeze going?

I live in rural Newfoundland. I’m surrounded by forest and we have two seasons, winter and summer lol I forgot to mention we will using a wood stove for heat and there’s the question of where to put it. I was thinking in the corner of the living room.

Just a few General comments/ questions

Normally when designing a floor plan bathrooms are aligned so that plumbing and ventilation are aligned with each other. So you might want to rethink locations of first and second floor bathrooms to achieve this.

However ?? - with no running water or electricity how will the bathrooms function? Chemical toilets? Or is the goal to add water and power in future?

You might want to rethink doorway to main bedroom on first floor. Since it opens to the living room and also gives a nearly straight sight line from front door you might find that lacks privacy.

Electricity will be run by a generator and water will be provided manually. The type of toilets in the bathrooms will be the kind with the pedal (similar to what’s in a RV) 5gl buckets of water will be on hand for washing up, using toilets, washing dishes, etc. I’m actually debating on having a bathroom on the top floor at all.

I totally get living off the grid but honestly you need water IMO no matter what. As for the actual design a long time ago friends built a thermal envelope house it had a gree house facing south with no windows on the N side and the spac below the greenhouse was filled with gravel the system basically allowed the gave to heat during the day and release the heat into the house during the night. I think the design came out of Mother Earth News and it was called a double envelope house. They lived in BC and the cost for heating in the winter was ridiclously low with only a wood stove to heat quite a large house.

A four bedroom tiny house? Ambitious! For a tiny house needing four bedrooms, it’s well laid out. I espe like that you added a second living room for the kids upstairs. Good luck with the project!

Have you considered a metal roof and collecting water in a cistern? We do that in the Caribbean (different climate, for sure). Ours is entirely gravity feed, except for pumping to a header tank for pressure.

Bedroom 2 downstairs looks very long and narrow. Think about where you will put beds and closets.

Are you planning to add solar power in the future?

With 4 bedrooms total a 1/2 bath on second floor makes sense - and will be appreciated by those making night time trips to use toilet without having to traverse stairs. Even though you will be living simply, it makes sense from a practical viewpoint.

One more thought - you might consider eliminating the indentation on back wall of primary bedroom and squaring it off the other bedroom. Often times it ends up being more cost effective overall —- More SF, cohesive roof line, simpler foundation. Something to consider.

When deciding upon the size of a room, you must consider what furniture will be needed for the number of people occupying the house. Adults don’t sleep in bunk beds, but kids may not want to have bunk beds forever, either. Plan on two twin beds per child’s room. Plan on two extra long twins OR a Queen or King bed for two adults’ bedroom.
I would put a 3/4 bath (add a shower) on the second floor so you won’t have people waiting in a line to use the downstairs shower. The downstairs bathroom could have a tub/shower combo or just a shower.

I should also add we will not be living in the cabin. It’s just for weekend getaways. Closets are not needed, just a spot to put linens.

You will need to consider how clean water and waste water are transported into and out of the house. If you want enough water for five people to take one (heated or unheated) shower per day for two days on weekends, that gives you a starting point. Having a bathroom on the second floor may be workable if you have an attic cistern. Also, your waste water will need careful design handling for septic field or other disposal routes that don't contaminate the local streams.

Do you have current experience with off grid water technology? If not, hiring an expert to consult would be a good idea before you settle on bathroom locations upstairs, downstairs, or both.

. . . I should also add we will not be living in the cabin. It’s just for weekend getaways.

. . . 5gl buckets of water will be on hand for washing up, using toilets, washing dishes, etc.

Weekends only it's way overdone. The focus should be on the congregration areas, you can reduce the number of bedrooms. A main bathroom and a powder room can be grouped near the Kitchen plumbing on the main floor. The kitchen looks the size of a standard house, yet you don't have the power or water to support that. The roof should be considered for runoff collection and future solar orientation and slope. I think you can also do better than 5ga buckets.

You need to rethink the entry way taking into account lots of snow/ wet boots / coats etc. I have similar entrance in our cabin and it is tiring to have the corner piled high with winter stuff and it spreads into the living area. Also you will need a direct path to interior wood storage and stove. You will be surprised by the amount of wood chips etc that gets tracked in every day. You will not want it tracked across your living area. Think carefully about the plumbing. Each weekend you will need to fully drain the drain pipes even though you are planning to use water buckets to fill the sinks.

I agree with Patricia’s comment about water.
Even if “weekends only” I would at least plan for infrastructure for future basic amenities. Not only from the perspective of your own enjoyment but for resale potential. So much more cost effective now to include now in the build stage than to add later.
There is a lot of good practical advice offered in the above posts.