We have one nice brooder house (the milk house next to our old dairy barn) and two horrible old ones that are supposed to be pasture houses, but were pressed into service more or less at random.

We're replacing the two horrible old houses with one big new one, building it on a pair of concrete slabs that have been here for decades (which, oddly, touch each other but are not at the same level.) Here are a couple of pictures of the brooder house under construction:

You can see the horrible old brooder houses in the background of the second picture.

Features of interest:

• We're using three courses of concrete blocks to make the house rat-proof and rot-proof, even with more than a foot of deep litter on the floor. This is essential. Not that we have a rat problem all the time, but even "once in a while" is way too often.
• We found a four-foot-wide exterior door, which makes it easier to get a wheelbarrow into the place.
• The three windows wouldn't provide anywhere near enough ventilation for a henhouse, but this is used solely as a brooder house, with the chicks removed to pasture houses once they no longer need heat. Smaller openings are adequate. (See Fresh-Air Poultry Houses for a complete treatment of this topic.
• A brooder house can be designed so it can be used later as a shed or studio or whatever kind of outbuilding strikes your fancy. In this case, the two-level floor would be a bit of a nuisance, but that could be fixed with more concrete.
• It's as close to our house as we can reasonably make it. It's good to be able to hear a commotion in the brooder house without going all the way out to the back forty.
• We'll be insulating the roof. This isn't strictly necessary in a well-ventilated brooder house, but is a nice touch.

[Here's a brooder house update, showing the house in a nearly-finished state and giving some more helpful hints.]