"It's ideal for Headquarters, of course. My father put every security measure known to wizardkind on it when he lived here. It's unplottable, so Muggles could never come and call — as if they'd ever have wanted to — and now Dumbledore's added his protection, you'd be hard put to find a safer house anywhere." —Sirius Black

I understand that 12 Grimmauld Place is protected by the fidelius charm, and invisible to the neighborhood residents. It is explained in the books that the neighborhood residents just accepted that there was an error in the numbering of the homes on the street.

I assume (possibly incorrectly) that the fidelus charm gives it additional layers of security above and beyond the normal "unplottable" enchantments. But just how well protected is it?

Let's say one were to walk a measurement from 11 Grimmauld to 13 Grimmauld, and got a width of 200' - and then drove the same path with an odometer - would the measurements be the same? If one were to use maps to measure distance between points, would they match the same real-world measurements? What would keep a muggle from noticing an incredible logical discrepancy in measurements?

  • baruffio.com/docs/unplottable.html - It doesn't make sense unless you believe in magic – Valorum May 21 '14 at 18:53
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    How hidden is what? – oɔɯǝɹ May 21 '14 at 19:25
  • @oɔɯǝɹ "How hidden is 12 Grimmauld Place?" the whole building is protected and unplottable. – phantom42 May 21 '14 at 19:53
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    @phantom42 hint: it's a joke :-) – oɔɯǝɹ May 22 '14 at 0:51

Harry thought, and no sooner had he reached the part about number twelve, Grimmauld Place, than a battered door emerged out of nowhere between numbers eleven and thirteen, followed swiftly by dirty walls and grimy windows. It was as though an extra house had inflated, pushing those on either side out of its way. Harry gaped at it. The stereo in number eleven thudded on. Apparently the Muggles inside hadn’t felt anything.
Harry Potter and the Order of the Phoenix, Chapter Four - Number Twelve, Grimmauld Place

Based on that description, I don't think there would be any logical discrepancy to notice. If you weren't made aware of its existence by the Secret Keeper, there'd be no distance to measure between 11 Grimmauld Place and 13 Grimmauld Place. Similarly, if the house is unplottable then it wouldn't be shown on any city plans, maps, etc. Looking at those would just show 11 and 13 Grimmauld Place immediately adjacent to each other.

It seems as though it should be as if the building had never been built, both in what people can perceive and what's displayed on maps, plans, etc. If the fronts of 11 and 13 Grimmauld Place are each 100ft wide, then the distance measured and the distance shown on maps should be 200ft. You wouldn't measure 200ft if you were there in person, but then see a distance corresponding to 300ft displayed on a map for numbers 11 and 13.

  • "Looking at those would just show 11 and 13 Grimmauld Place immediately adjacent to each other." Right, but if the map shows that it's 200' from edge to edge of 11 to 13 Grimmauld (with 12 being unplottable), is that the same distance that one would measure in person? Is the 200' on the map the actual number, or take into account the unplottableness? – phantom42 May 21 '14 at 17:25
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    @phantom42 I think - though I'm afraid I'm not aware of any canon evidence to back it up - that the distance on the maps should take into account the "unplottableness" of 12 Grimmauld Place. I'll edit to address that particular part more clearly. – Anthony Grist May 21 '14 at 17:36
  • Clearly the magic causes the matter to co-occupy different dimensional realities, as seen by this house, and things like the knight-bus. At least that is a sci-fi take on it (which is basically magic, just with a more complex explanation for it). – James Christopher May 22 '14 at 0:40

They never really go into the absolute specifics of invisibility & illusion in the HP books, nor expand on them in movies. One option to be similar to this are two (British, actually) instances of this - the Perception Filter from Doctor Who/Torchwood, and the SEP (Someone Else's Problem) Field from the Hitchhikers Guide series - in both of those cases, the location is physically there, but no one can see, nor care, about it. Muggles walking by with an odometer would think that the stupid odometer is acting up again, and stalk away muttering about the equipment budgets.

The other option is the idea of shrunk/crushed or extra-dimensional space. The quote in the books is "It was as though an extra house had inflated, pushing those on either side out of its way" - but then, that is the view from a fifth year wizard rather than an enchantment specialist. There are similar examples in the Weasleys Mansion-tent, and Moody's (or was it Junior's??) Bigger-on-the-inside Trunk. There are also mentions of objects being shrunk by magic as well. In this option, there are three options:

  1. 12 Grimmauld Place exists solely in extra-dimensional space, similar to Hermione's bag. When it "deflates" it is effectively "gone" in the physical world. Conservation of Mass be damned.
  2. It shrinks partially. It could be that just the front entrance shrinks (so the front of the building is technically narrower, 200', than the back of the building, say, 250', but appears to still be squared at the corners). This slightly alluded to in the movies, where HP & co hear a crunching/scraping noise when the front of the house is appearing - I do not remember if that is reflected in the books. Alternatively, it shrinks all non-occupied rooms to Conserve space for the occupants while bending physics rather than breaking them, similar to the room of requirement.
  3. It shrinks completely. If you walk by with an odometer while the front is closed, you'll have the 200' distance (using the above example). If you walk by while the front is open with the odometer, you'll have a larger width (again, 250'). Occupants on the inside are also shrunk, but relative to the interior, so the Order of the Phoenix do not view there is any change.
  • Stephen King has a similar concept, which he uses in The Eyes of the Dragon and the Dark Tower books — people or things can be dim. It seems to work like JKR’s muggle-repelling charms: “When one was dim and a person approached along a passageway, one simply drew aside and stood still and let the person pass. In most cases, the person’s eyes would drop to his own feet or suddenly find something interesting to look at on the ceiling. If one passed through a room, conversation would falter, …. Torches and wall sconces grew smoky. …” – Peregrine Rook Jun 10 '16 at 5:14

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