As land prices rise, home builder seek out ways to optimize the lots they have by increasing density, especially in urban areas. Our recent roundup shows some of the best ways that builders are doing more with less when it comes to land.

But in some crowded international cities (with lax zoning requirements), builders and architects have been able to stack and squish housing into places where it seems nearly impossible. Scroll through to take a look at these examples of high-density housing on steroids.

Crowded House
You’ve heard of crowd funding, take a look at crowd building. French architecture firm Périphériques Architectes designed this prototype (above) as a way of making Paris affordable by putting the power of building into the hand of residents. The architects envisioned the structure as a mix of different sizes, types, and styles of building, according to this article in Inhabitat.

Blending In
Hillside homes in Iran and Tibet almost look like they are part of the landscape. You know it’s high-density housing when one family’s roof is another’s porch, according to this article in Picture Correct.

Skinniest House in the World
Architect Jakub Szczesny has taken tiny home and infill development to a new extreme with Keret House in Warsaw, Poland. In a space between buildings where residents used to leave trash or old furniture now resides a triangular-shaped home with a width that ranges between 36 inches and 48 inches. Tech Insider reports that the home is accessible via drop-down stairs that leads to a “latch” in the base of the first floor. To travel between the home's three floors, there's a single ladder in the hallway. The bedroom on the third floor squeezes in a bed between the walls and has a built in desk that takes up nearly half the width of the room.

Maximum Occupancy
This article from the Huffington Post mentions Amsterdam’s famous Inntelhotel, an eclectic structure that stacks 70 houses on top of each other.

Too Close for Comfort?
Hong Kong is filled to the brim with over 7 million people, and with land at a premium, the city builds up, not out, according to this article in Inhabitat. Residential towers are stacked tightly together often reaching 30-stories or more. At its peak, residential density tops out at 110,100 people per square kilometer (compared to 59,150 in New York and 27,100 in London). German photographer Michael Wolf has captured the dizzying array of apartments in his series of books, Architecture of Density.

Urban Infill
In a new way of thinking about urban infill, architect Stephane Malka sees opportunity in the gaps and vacant spaces in between Paris buildings. Named “3box,” the units are suspended and raised in between two buildings on a corner plot. Read more about it in this Design Boom article.

Hard Rock Hotel?
It's no joke: A luxury hotel in China will be built into a rock quarry. The project will encompass the construction of a five-star, 383-bedroom hotel built into the side of a disused, water-filled quarry, according to its designer, international engineering firm Atkins.