>>Has anyone here built a DIY out house for snakes?
No, but I've modified two existing structures to house reptiles. One was a "pole barn" or one of those metal sheds.
The other was an unheated room in a garage.
I've learned a lot from both. I'll likely be building an outbuilding for my snakes in the near future once I buy a house so feel free to stay in touch.
For heating I recommend those oil-filled, radiatory style space heaters. They are remarkably efficient and very safe. There are also radiant ceiling heaters but they are more expensive.
I run mine through a thermostat rated for 2000 watts and maintain a constant temperatures. I run a multi-strip outlet through the thermostat so I can plug both the heater and a box-fan into the strip. What this does is kick the fan on whenever the thermostat cycles. This way the fan distributes heat only when the heater is running. It also provides me an audible indicator of how often the thermostat is cycling.
Now I also have an air filter plugged into that thermostat.
Thermal gradiants and night time temperature drops are provided to each cage individually when appropriate.
If you'll need any cooling for brumation purposes or during summer heat waves I recommend a portable air conditioner vs. a window air conditioner.
They're more quiet and only need to be vented through a very small area.
Insulating the floor is a very good idea unless you have a lot of species with different temp requirements. In that case it can be helpful to have some near the floor and others higher up. Or some species can be brumated on the floor while others still have the benefit of a heated room.
Another way around insulating the floor is to install radiant floor heat when the slab is poured. You can use either electric panels or a water based system. I doubt either is practical from a cost standpoint, especially in southern california where there are probably not incentives for their use.
When you insulate consider using an insulation with a reflective component as well as a resistive component. These are much better for the type of heat typically used to for reptiles. They will also do a better job of preventing the building from heating up on sunny days.
Lastly, find out what the legal ways are for you to run water. I currently cannot do that and have to carry 4 gallons of water to my snake room every time I clean. For regular water changes I just use dilute Nolvasan. About once a month I do a more thorough job and bring all the bowls inside.
I wish I had water.
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)