Puppies are very fragile. Especially in the first few days of their lives. Pups need a consistent temperature controlled area, where the mom feels safe. For some dogs this is in your house, but if you have a noisy, chaotic house, moms are not going to be as tolerant of this when babies are here. Also, birth is a very mess business. When only dealing with one or two dog, you can manage in your home, but Canine Corporations require a dedicated Nursery.
The main reason that you should not have a doggy door near the puppies goes back to the temperature control. I prefer mom's in a sizeable area where the mom can have the bed of her preference, and a large area to run around and use rest room. Therefore does not need access to the outside.
You should definitely utilize a heat lamp, but it should be aimed on part of the pen. I prefer near the door of their whelping box. That way mom can move closer to the light if needed or away if getting too hot.
Many notice that my moms do not get a heating pad or blanket. This comes from experience. When a female is nesting, and having/had pups, they tend to dig and toss and turn. Heating pads can catch fire or electrocute mom. Pups can get wrapped up or under either of those things and get smashed/suffocated. Again, our motto is safety over human comforts. Plus, the slick plastic bottoms are easier for moms to keep clean.
Access to the nursery should always be limited. Visitors, children, workers, etc. should always be monitored or not allowed at all. I once had a mom kill 3 of her 5 week old puppies when I had a worker installing some new plumbing. And this was a mom who I would have never expected that from. If you must have work done any where near your nursery, pull all moms from pups while work is being done.
I prefer multiple small rooms that house only one other mother. If you have a room with more pens, then use that ready to wean or moms not quite due. But try to have at least one contained room that is controlled for actual birthing time. You also may want this room to be warmer than you keep the rest of your facility.
So Phase 1 is in your home. But once you grow past a few females it is time to start your phase 2 nursery. Start building on just a plain slab of concrete. Insulation, temp controlled exhaust fan, and ability to bleach are requirements. Remember to start small, but make sure you have room to expand.
This video shows you my current nursery, but my dream nursery will be built soon!