Just started really thinking hard about spell design and came up with a travellers type spell for shelter. Along similar lines to a post on Leos Tiny Hut.
Hoping to make this a lesser enchanted Device : Pavilion of Marvellous Repose
CrHe: Creates large pavilion made from plant products (Base 2, +2 intricacy, +2 sun, +2 Room) : Level 30
Creates a large tent, made from canvas with poles & ropes. Interior is warmed or cooled by a ReIg effect. Can sleep 100 people of size 0. Beds are thickly quilted & well made as well as being comfortable.
R : Touch D : Constant T : Room
(+2 Sun, + 3 environmental Trigger)
ReIg: Regulate temperature from cool to very warm : (Base 1, +2 Sun, +2 Room) : Level 20
R : Touch D : Sun T : Room
Creo spell must use T:Ind or T:Group. So you can't use T:Group for your spell.
An Individual for Herbam is about 1 pace by 1 pace by 1 pace. Then you have to add magnitudes for size, muliplying the volume by 10 for every magnitude added. Even if your volume is mostly empty, you must still account for it.
A tent large enough to sleep 100 people is huge. It takes about 6 by 2 feet to have a man lie down. So your tent is 1200 square feet at a minimum, or 133 square paces. Even if it is a sprawling low tent (bedoin-style), it must be at least 3 paces high, for a total of 400 cubic paces. So you'd need 3 magnitudes for size.
Also, canvas made into a tent is treated and processed, so that 2 magnitudes added instead of 1 (which you included in the Base).
So the result would be R:Touch, D:Sun, T:Ind using Base 3 (treated and processed), +1 Touch, +2 Sun, +3 Size, +2 complexity. Total level would be 35.
The warming effect should probably be a seperate spell, fort it has little to do with a tent. If your storyguide allow you to integrate both effects into the same spell, you'd need at least +1 magnitude for adding the effect in and +2 magnitudes for the Ignem and Rego requisites. Pushing your spell to level 50, just shy of needing to be a ritual.
I agree with Arthur in most cases on his recommendations for the spell, with one exception.
I'm not familiar with, nor can I find, a base 3 (treated and processed) and I'm not clear on what it means.
Adding complexity to a spell is also fraught with some subjectivity. Is it +2 or +1, or something altogether different? It's a judgment call, because there aren't any guidelines for determining complexity. The closest thing is a benchmark, Conjuration of the Mystic Tower has a +3 for elaborate design.
Don't forget, there needs to be a finesse roll made here, all items conjured should have an Int+Finesse roll made to see how well the conjuration conforms to the realm of forms where the perfect tent exists. It's probably a pretty easy roll for most magi, EF 6.
Size is in terms of mass not volume. You don't need to account for the empty space.
A failed roll will give you an ugly, but functional tent. Too many failed finesse rolls and people will start telling stories. And then there will be faerie Redcaps. And they will take your job. Do you want that? Damn, I was trying to argue that you didn't need to worry about the finesse roll...
Try T: Individual. For Herbam this is a plant about 1 cubic pace. Adding 1 magnitude multiplies this volume by 10, adding 2 magnitudes means you create an objcet made of plant products of 100 cubic paces. But this is a massive cube of wood!
Let's assume 1 pace = 1 m for simplicity.
Assuming the walls/roof/floor is 0,2 m thick on average (to make it easier, supports are likely to be wider while non-load-bearing surfaces are thinner), a 10x10 m hut 3 m high is made with 6 cu m per wall side and 20 cu m for floor and roof respectively, for a total of 56 cu m of material...that's less than 100, s0 we can almost double the length on one side (and assume a height of a little less than 3 m, to make things add up). We now have a 10x20 m building, or 200 sq m. Assuming a sleeping position takes up 1x2 meters you could have 100 people staying here. A lot more if if you make bunks in 3 levels (but here you need to account for walking space in between. Also is the spell needs to make the furniture as well you need to account for materials for this as well...but +2 mags for Intricacy should take care of this regarding complexity.
For a tent, even less material is needed for the walls etc. so in theory it could be even bigger.
So, Ind+2 mags for size is equal to Room, so the final level stays the same.
The CrHe guidelines state that a single spell generally only creates one type of plant product, so base 2 might be correct, but at least one magnitude should be needed to make all the different types of plant product you want. Size +2 sounds right, looking at the calculations done by Christian. This would make it L35. And then you need to add in the number of uses per day; whether you want an environmental trigger; and you might want to make it dismissable (Re req, I believe*, +1 magnitude), otherwise you can't take it down when you've finished with it. You could take the device away and leave the existing tent behind to expire, however that might attract attention from mundanes.
You can't put a second effect into a lesser enchanted item. And it would be more versatile to make the ReIg effect as a separate lesser item anyway, then you can use it on/in any room - however something is wrong with your level calculation here (base 1 +4 magnitudes = L5 not L20) and I think it should be Base 4 (control a fire (or, in this case, heat) in an unnatural fashion) +2 sun +2 room = L20. Again plus uses per day, etc.