Oh, Look. More Rooms.

A subtrope of Bigger on the Inside. A place looks fairly normal from the outside, and possibly even when you get in, it's of a reasonable size. But there's this door in the back. Open it, and... there's a whole new section of the place, easily as big or bigger than everything you've seen so far! And look - there's a door in the back of that too, which leads to yet another new section - or worse, five doors...

Basically, it's when Bigger on the Inside keeps happening to the point of an Overly Long Gag. Related to Big Labyrinthine Building.

Examples of Oh, Look. More Rooms. include:

Comic Books

When Shade the Changing Man moved into a crack in the pavement of Times Square, it was already Bigger on the Inside. New rooms appeared as the story required, and his son George spend months touring them.

Literature

Arguably Death's Domain in Discworld. The initial hallway is intimidating enough, but several of the rooms along it open up into cavernous chambers filled with books or hourglasses.
House Of Leaves. The house on Ash Tree Lane is this, having doors that open from the normal part of the house to a seemingly never-ending sequence of cold, colorless rooms, hallways, and stairs, just like a Clown Car Base.
The Room Of Requirement in Harry Potter.
Lady Door's house in Neverwhere. It's got a nearly-infinite number of rooms, scattered across all of space and time. There's no need for a physical connection between them, as her family has the ability to open portals by touching anything that opens and some things that normally don't.
John De Chancie's Castle series features Castle Perilous, a castle containing portals to 144,000 different universes.
Morwen's cottage in the Enchanted Forest Chronicles appears small on the inside, but has a door that leads to a different room every time it's opened.
The Godmothers hut in Mercedes Lackey's The Fairy Godmother looks like a normal cottage in the woods even from the first few rooms. Then you realize that there really shouldn't be room for two stories, a big kitchen, a pantry, a library, and several other rooms. Then, later in the book, Godmother Elena reveals the true nature of the totally not just a house. It's simply magic.
The titular House of Many Ways follows the trope description almost to a T, but the door at the back is actually the door in the middle. What starts out as a house with two rooms and one inside door turns out to have two rooms, a dark hallway, and another hallway with bedrooms and a washroom, then is eventually revealed to contain part of the brownies-by-any-other-name's underground caves and the entirety of the indoors of the royal castle a two-hour walk away.
Charles de Lint's Tampson House in Moonheart and Spiritwalk is a good example. It's a mansion that takes up a city block, but looks like a series of townhouses from the outside. That's without going into how one can reach multiple spirit worlds from within it.
The Neverending Story features a palace of doors which can lead to any portal in any world at any moment, though the entrance is constantly changing. It does work as a useful travel mechanism, though; each room has two unusual doors (like a giant zipper or a vault door), and to get to one specific place you have to go through the doors that remind you of things at your destination.

Live Action TV

Though they rarely bother to show it anymore, the TARDIS from Doctor Who has so many rooms even the Doctor himself isn't sure where they all lead (except for that time where they jettisoned part of it. He knows that door doesn't lead anywhere anymore).
- Has happened to a couple of houses in the new series, when aliens attached new rooms or floors to an existing building.
This happens in the first episode of The Goodies when Greame is showing the other two around their new office.
In The IT Crowd episode The Red Door, Jen suddenly becomes curious about the titular door in the corner of the office, which hasn't been shown on screen previously. At the end of the episode another, more terrifying door is briefly shown (and never mentioned again).

Music

"House of Four Doors", by The Moody Blues

Tabletop Games

In Dungeons and Dragons, justified by the existence of other planes of existence having magic portals to them, sometimes cleverly disguised as a normal door.
- Baba Yaga's Hut in three different Dungeons and Dragons incarnations: the original in the 1E DMG, "The Dancing Hut of Baba Yaga" adventure in Dragon magazine #83, and the 2E module The Dancing Hut of Baba Yaga.
This can be a power of Manses, buildings that channel geomantic energies, in Exalted.

Video Games

Peach's Castle in Mario & Luigi: Bowser's Inside Story. The initial castle area is large enough, but as you progress you keep opening up more and more areas, until it turns out to take up well over a hundred screens. Compare this to the entire Toad Town, which takes up five.
Super Mario 64, perhaps the ancestor of the Bowser's Inside Story example above. The room you walk into when you enter the castle seems to be about the size of the castle. However, more and more places open up. The basement makes sense. It could be under what we see from the outside. However, the upper levels are definitely an example of this.
The player's house in Animal Crossing, once you start adding on extensions.

Webcomics

Under Lock and Key, a webcomic whose entire premise is this; every door can open into a totally random room in the house, and nobody knows how far it goes.

Web Original

SCP Foundation-167, the aptly-named Infinite Labyrinth. There's also SCP-184, which does this to any building it's placed inside.

Western Animation

The Classic Scooby Doo thing where they are running from the bad guys, hiding in barrels or popping in and out of doors, often uses this.
In Futurama, Fry spends a whole episode living in Bender's 'apartment' - a room just big enough for two people to stand up in - before discovering a door in one wall which leads to the closet, actually a spacious apartment with several further rooms leading off it. The door wasn't even being hidden by clever use of camera angles or anything - it was a blank wall in all previous scenes.
In King of the Hill, the Hill family takes a vacation to Japan. In their tiny hotel room, they do all their cooking and sleeping in a space about half the size of their living room, as a parody of overcrowding in that country. However, at the end of the two-parter when Hank's brother comes to help them check out, it is revealed that that was just the sitting room; there was a sliding door leading to several other rooms, and even a fountain.

Real Life

The Winchester Mystery House. A huge, sprawling mansion with hundreds of rooms. At one point you think you've reached the end, but then you open the closet door and it opens on a whole new wing.
42.zip, while mentioned on the Bigger on the Inside page, probably fits better here. It's a .zip containing 16 more .zips, which each contain 16 more .zips, and so on and so on. At the very bottom of the chain is a 4.3GB file, meaning altogether it takes up 4.5 petabytes. That's more than the entire Internet Archive and all the 3D rendering effects for Avatar combined.
It's an old joke about housing in New York City that everyone is always secretly hoping they'll notice a door one day that they never saw before, and it turns out that their apartment is twice as big as they realized.
Many used book shops are like this, since they tend to be built into old houses. For example, one has a main room that many people never get beyond... despite the fact that beyond it, through a tiny opening, there are five more rooms of equal or greater size.
Cave exploration often adheres to this trope, as a previously-documented cavern might turn out to have another huge series of chambers and tunnels attached to it when rubble is cleared or divers investigate a subterranean pool's depths.