We have built a "treebog" which is probably the simplest form of compost loo, allbeit slightly more tricky to build. Although no need for it to be fancy. No need to empty it and for hutting use I very much doubt it would ever fill up! The concept is simple: raised toilet seat with area underneath where everything falls on the ground, screened with straw or something so you don't see it. Plant willows all around the loo and they soak up all the nutrients and kind of empty it! You can coppice the willows every year or two. Plenty of articles on the web.
I'd put another vote in for a basic bucket and sawdust toilet. We renovated an old caravan last year and went for this option. It's been great so far, very cheap and easy - £8 plasterer's bucket, a brand new soft close seat for £10 (it is lime green...) and a few bits of plywood. The kids love adding the sawdust, no unflushed toilets anymore!!
When I finally build a hut, I'll be going for the same setup.
The toilets in use at our site so far are very simple composting toilets. A box with a toilet seat and a urine separator. My box was built by a joiner for about £100 a decent urine separator cost £85 which just screws onto the underside of the seat. The poop and sawdust goes into a 25litre bucket and the urine into a separate container for more regular emptying. Or the urine can be directed to a container outside or to a soak away. I then have a compost bin outside, half filled with straw, into which I empty the poop bucket occasionally. The urine gets emptied into the compost bin every day. Urine can go into the bucket with the sawdust and poop but it will fill up quicker and you will need a bit more sawdust to prevent the rabbit hutchy smell. I then cover the compost bin with another layer of straw and pull it aside when I empty the bucket again or when I add any other compostables like veg scraps. Should take years to fill and when it does I'll get another and eventually empty the original and probably use it in my garden.
Building Control (Dumfries and Galloway) told me I wouldn't need a building warrant for grey water soak away but would need one for a composting toilet and I got the feeling that they had little knowledge on the subject so told them we would use camping toilets which obviously we haven't. I hope that if they come looking I will be able to demonstrate how simple and clean they are.
Humanure book mentioned above by Joseph Jenkins is brilliant
Wee Hooses provide compost toilet kits and parts
I do have a bucket with a lid that seals but I don't use the lid. The bucket is for poop, sawdust and paper, is pretty dry and doesn't smell. I left mine half full from last summer and only emptied it recently. If you dont let it get too full its not heavy to carry. Or you could have several with lids and set them to one side untill you have means to transport them. The buckets are pretty cheap. I get empty dairy chemical bucket from the farm for free. I have an old 5 litre container with a handle for the urine which I empty into the compost bin regularly. Others on the site have the urine diverted to a soak away which makes more sense if you live a distance away as you wouldn't want to forget to empty the container! That would get smelly. You can let everything go into the same container and use a bit more sawdust but it fills up quickly and can smell a bit like a rabbit hutch. Also if you put a lid on a bucket that contains urine too, I find you get condensation and black mould grows on the underside of the lid which s a bit yuck. Maybe more sawdust would solve that
This one is good value and UK made: https://freerangedesigns.co.uk/collections/compost-toilets/products/urine-separator-for-compost-toilet-3
Check out the book 'the humanure handbook'. Really Useful
The only requirements (that I'm aware of) stated in planning at the "encampment" was that an area was designated for it, and that cartridge/cassette toilets were used...its something that still has to be discussed in more detail though
I doubt they will be familiar with vermiculture...yet! Seems some ask for details of the type, spec sheet from the manufacturer, and some don't raise it at all.
Are there particular composting methods that planners like? ( are familiar with)
A bit more info on Vermiculture toilets here...
I think looking at the set up these might only be suitable for a good sized plot, not where there are a number of huts at relatively close quarters. Additionally, the question of where the waste liquid goes would have to very carefully considered.
It depends very much on your budget, and the space available inside and outside. Vermiculture Composting systems are flushable so the same as a regular toilet to use. Comrie Croft has them in their meadow camping area and say they work really well, though correct sizing is important to keep the worms happy. They need a sloping site and a fair bit of space outside for the composting tank and soakaway.