Not sure what you mean by "large" or "built into", but a coupla years ago I built the thing that Robert Plamondon describes as an insulated hover brooder (go to
www.plamondon.com, go to his chicken articles, then hunt around til you find it; further construction details are given in his 'Raising Baby Chicks' book) and use it in my coop.
Mine is 2'x4' but you can build them larger of course, in fact his plans are for a 4x4 one. It is like a low table legs and hover-style partial sides. You can prop the legs up on bricks as the chicks grow if you need more clearance.
It is annoying that (because the thing is so low to the ground) you cannot easily see the chicks under it without either kneeling in pooey litter or lifting the thing up on one side -- I periodically think about replacing one side wall with plexiglass but I never actually DO it, LOL -- but other than that, three thumbs up.
The nice thing is that you can use whatever pen you have free, or even the end of the aisle walled-off, because it is like a table and can be put wherever you want; and then when you are NOT using the brooder it can be stored elsewhere so you have that space available for the now-grown chicks or whatever else. The brooder does still take a certain amount of storage space, but not as much as if you had it permanently built there so it was ALWAYS occupying coop space.
JME, good luck, have fun,
Pat