I have a detached building that I use for my lizards in the winter.
A few thoughts for you.
Firstly- if things are fairly tight and well insulated, you may find extra heat unnecessary. My building is fairly large (42x30') but is well insulated and pretty tight. As a result, I never had to have heat other than the heat lamps during the day and then nothing at all at night. We have temps in the 20s and teens at night in the winter pretty often, and occasionally single digits with windchills below zero (25 below once last winter but that was extremely rare). Things remain above 50 over night in my building without any heat. Which is safe for all chameleons I've ever heard of. I'm not sure how much of that is the size of the concrete pad and good insulation though. I would think if you have a concrete floor and a well insulated tight small room, you may find yourself in a similar situation on a smaller scale.
And that kind of brings me to my second point- If it is small and well insulated and you have several cages, you may find heat lamps do the job during the day for ambient in the room. They do in my building- although mid winter ambients sometimes only climb into the mid 60s for a month or so, and the lizards get a little winter cool down, while still having access to heat lamps.
And then my third point- the building remains rather warm at night now (upper 60s to low 70s- higher than really I want) after adding lots of heat tape on shelves to try and increase production of feeders last winter (jury is still out on whether I see much of an increase vs ambient room temps on the roach species I'm breeding- I really am not sure I have seen a difference in overall production this year vs years past although this winter will prove the test- I didn't have many roaches last winter to compare with previous years and winter production always slowed down quite a lot). If you are planning on doing something similar- roach bins on top of a shelf of flexwatt heat tape, you may find this provides plenty of room heat day and night anyway in a room that small.
And my final point- Spring and fall temps are difficult here. I have an attic fan on a thermostat in one wall with leuvers (sp?) on the outside to vent excess heat in fall and spring. I've found excess heat to be more of a problem than not enough heat in my situation- I can count on fighting heat spring especially and fall but never have to fight cold.
