Threshold vs magic

"Come on, Dresden," Rudolph snapped. "The lieutenant is waiting."
"Is Mrs. Malone here?" I asked.
"Yes."
"Go get her. I need her to invite me in."
"What?" Rudolph said. "Give me a break. Who are you, Count Dracula?"
"Drakul is still in eastern Europe, last time we checked," I replied. "But I need her or Micky to ask me in, if you want me to do anything for you."
"What the hell are you talking about?"
I sighed. "Look. Homes, places that people live in and love and have built a life in have a kind of power of their own. If a bunch of strangers had been trouping in and out all day, I wouldn't have any trouble with the threshold, but you're not. You guys are friends." Like Murphy had said - this one was personal.
Stallings frowned. "So you can't come in?"
"Oh, I could come in," I said. "But I'd be leaving most of what I can do at the door. The threshold would mess with me being able to work any forces in the house."
"What shit," Rudolph snorted. "Count Dracula."
"Harry," Stallings said. "Can't we invite you in?"
"No, Has to be someone who lives there. Besides, it's polite," I said. "I don't like to go places where I'm not welcome. I'd feel a lot better if I knew it was all right with Mrs. Malone for me to be here."
Rudolph opened his mouth to spit venom on me again, but Stallings cut him off. "Just do it, Rudy. Go get Sonia and bring her back here."
Rudolph glowered but did what he was told, going into the house.
Stallings tapped out a cigarette and lit up. He puffed for a second, thoughtfully. "So you can't do magic inside a house unless someone asks you in?"
"Not a house," I said. "A home. There's a difference."
"So what about Victor Sells's place? I hear you took him on, right?"
I shook my head. "He'd screwed up his threshold. He was running his business out of it, using the place for dark ceremonies. It wasn't a home anymore."
"So you can't mess with anything on its own turf?"
"Can't mess with mortals, no. Monsters don't get a threshold."
"Why not?"
"How the hell should I know," I said. "They just don't. I can't know everything, right?"
"Guess so," Stallings said, and after a minute he nodded. "Sure, I see what you mean. So it shuts you down?"
"Not completely, but it makes it a lot harder to do anything. Like wearing a lead suit. That's why vampires have to keep out. Other nasties like that. If you give them that much of a handicap, they have trouble just staying alive, much less using any freaky powers."

Grave Peril Chapter 12, Page 109-110


I got it then. Murphy wasn't going to ask me in. There are plenty of monsters running around in the dark that can't violate the threshold of a home if they aren't invited in. One of them had caught up to Murphy last year, nearly killing her, and it had been wearing my face when it did it. No wonder she didn't look exactly overjoyed to see me.
"Murph," I said, "relax. It's me. Hell's bells, there isn't anything that I can think of that would mimic me looking like this. Even demonic fiends from the nether regions of hell have some taste."
I stepped across her threshold. Something tugged at me as I did, an intangible, invisible energy. It slowed me down a little, and I had to make an effort to push through it. That's what a threshold is like. One like it surrounds every home, a field of energy that keeps out unwanted magical forces. Some places have more of a threshold than others. My apartment, for example, didn't have much of a threshold - it's a bachelor pad, and whatever domestic energy is responsible for such things doesn't seem to settle down as well in rental spaces and lone dwellings. Murphy's house had a heavy field surrounding it. It had a life of its own; it had history. It was a home, not just a place to live.
I crossed her threshold uninvited, and I left a lot of my power at the door as I did. I would have to really push to make even the simplest of spells work within. I stepped inside and spread my hands. "Do I pass inspection?"
Murphy didn't say anything. She crossed the room and put her gun back into its holster, setting it down on an end table.

Summer Knight Chapter 7, Page 91-92