Chickens house inside a barn

My partner and I have sketched different ideas to keep our chickens warm in the Norwegian winter. The problem is that we have lots of uncertainties about chickens needs which makes it hard to choose the design.

We are moving to a smallholding in three weeks, where there are 7 chickens (they're "older" 3 years old chickens gifted from a neighbor who didn't want them longer). This will be our first winter together. I gathered much inspiration from reading many posts on permies and Paul's article.

The chickens live in a room in the barn constructed with big logs and it's pretty spacious, too spacious for 7 chickens and a small heat lamp too heat. What we want to create is a smaller space inside this room so that the warm lamp will be effective in warming up the smaller area.

We want this space to be an inviting space for the chickens to roost and sleep and "chill" on the coldest days. I'd love to hear your thoughts on the following:
1) which size do you think is appropriate? Balancing between effective heating but enough space for them to exist in there if the bigger outside room is too cold
2) which height is appropriate? Giving that they have access to the bigger room (and also to the outside when appropriate)
3) should they have water also inside this smaller house (plan is for them to have free access to the outside room where there would be water available - given that it won't freeze)
4) do they need a sand bath space in the smaller room? Is it enough to have it in the bigger room?
5) what else is necessary for them to have a good winter?
6) is it okay - both for them and for energy efficiency - to have an open floor in this little house? We were maybe just thinking poles they can jump on as a "stairs"
7) would a design as simple as 4 walls made with bales work?

That room looks like a good size for 7 chickens.

Chickens need a place to roost at night.  Cut tree limbs attached to 6 inches from the walls of that room would work.

Chickens need nesting boxes to lay their eggs.

In addition to food chickens need water and grit.  The grit can be as simple as dirt or even oyster shells.

I am sure other will chime in as to their suggestions.

I like to work out a system where the chicken door can be opened and closed without stepping inside the chicken enclosure.  I try to also work out a means of filling the water and food without  going inside.  On those days I am in a hurry, it really helps.

I would not use a traditional heat lamp. You could look for an alternative safer option made near you or make something. I made a hanging wire rack that is parallel to the ground and covered it with reflective bubble wrap, with flaps down the side. Mine was in a mobile coop and for meat birds but they loved it. It requires no electricity and has no risk of fire. You could create similar contraptions that allow chickens to stay warm without electricity. Induction brooders, barrel heaters are good options that do use electricity but are far safer than a heat lamp.

Just something to consider and not directly addressing your question: My reading says that using a heat source makes them dependent on it and if you lose power, they will be greatly harmed during your deepest cold. But you could supply them with a ventilated but draftless habitat that forces them to acclimate to the cold and they can handle the occasional dip to -40 and are only at risk of frostbite if they're a variety with big wattles and combs.

I keep my chickens in an unheated building. In winter, the temps inside the henhouse dipped to -20°C [-5°F]. The chickens grew enormous amounts of down and feathers.  They were enormous!! They happily slept huddled together on the roost. In the daytime they pottered around in the snow.
Chickens can be extremely hardy, but you need to expose them to cold - or keep heating the henhouse all the time. They grow fluffy only if they have a reason to.

As others said, heat is entirely unnecessary for chickens.  I have had temps to -40F, and I get -20 to -30F every year.  My chickens are fine.  Chickens need a well ventilated area so that no humidity whatsoever accumulates.  They need a draft free area to roost.  I like flat wide roosts so that the birds feet stay on top of the roost rather than wrapping around.  I use 2x4s with the 4" side up.  The reason for that is that when the birds roost, they cover their feet with their bodies.  The coop should be bone dry all the time.  You have a large are so having water available in the coop isn't a problem, but I don't ever keep water in mine, only outside in their covered run.  I would advise you not to bother making your "room in a room".  Your birds will be perfectly fine without it.  As a new chicken owner, it's very hard to take this advice, but I can tell you, in my experience, this is the best way to help your chickens in the winter.   This book explains everything you need to know about keeping chickens in cold weather:  Open Air coop book

I built and use this type of coop after using several other kinds, and I will never go to another type of coop again.  My birds are healthier and happier than ever.  You will see that this type of coop has an entirely open front that stays open the entire winter.  You don't need to move to this type of coop of course.  Your barn has plenty of good open space and air for your chickens, but the other information in the book should put your mind at ease about your chickens and cold weather.  

Best of luck to you!

This is the henhouse. It has a 500 square foot run plus the house with a foot of substrate and lots of roosts up high.

If you care about the barn at all you should reconsider not using a heat lamp.  Around here whenever a barn burns down it is usually caused by a heat lamp.

What is the coldest temperature your area sees in an average winter?

Just another picture of chickens in the house that dropped to  -20°C