Arriving in New York City often entails learning a new vocabulary, and this holds especially true when it comes to housing. From acronyms such as EIK (eat-in kitchen) to euphemisms such as “open-concept living/dining area” (this means that the kitchen is in the living room or that there may be no living room at all), new city residents generally have a lot to learn. But perhaps the most common housing jargon used in New York City are the adjectives “convertible” and “flex.”

What one soon discovers—often only after a disappointing apartment viewing—is that “convertible one bedroom” or “flex one bedroom” technically refers to a studio, and likewise, a “convertible two bedroom” or “flex two bedroom” technically refers to a one bedroom. Unlike most studios or one bedroom units, however, these convertible or flex options are theoretically large enough to be converted into units with an additional room.

Until 2005, flexing an apartment, which once entailed installing a pressurized wall (nearly always at the tenant’s own expense), was a common practice. But the climate changed after a devastating 2005 fire in the Bronx that killed two firefighters. An investigation into the fire revealed that illegally divided rooms were largely to blame for the fatalities, since the illegally divided rooms had confused the firefighters, preventing them from escaping. Since the 2005 fire, which resulted in the building’s former and current owners along with two tenants being charged with manslaughter, many owners and management companies have put the brakes on tenants installing temporary walls, but this doesn’t mean that flexing has stopped. In fact, temporary wall companies appear to be thriving just as much as they were over a decade ago. One difference is where pressurized walls were once the go-to option for flexing an apartment, sliding walls, sliding doors, and freestanding bookshelves are now the preferred way to flex.

1. Is flexing an apartment still legal?

Technically, if you want to alter your unit, you need to get the green light from the New York City Department of Buildings (DOB) and adhere to all the same criteria needed to install a permanent wall. In other words, if you want to install a new wall, you’ll need to contract with a licensed engineer or architect who will draw up a plan, have the wall installed, and then have a licensed engineer or architect inspect and approve the work in compliance with DOB standards. If you’re installing a wall to carve out an extra bedroom, it will also need to meet the legal definition of a bedroom in New York City—this means the room must be at least 80 square feet and have at least one window. Finally, if you think you don’t need a living area, think again. After the conversion, you legally must still have a living room and in most cases, it must be at least 150 square feet. Finally, if the creation of your new room in any way interferes with your ventilation or sprinkler system, it won’t be legal.

In theory, a tenant could—with their owner’s and management company’s approval—go through the expense, time, and cost of legally installing a wall, but of course, this is not how the vast majority of apartments are flexed.

2. What options are available for legally flexing an apartment?

Most flexed apartments are no longer flexed using a pressurized wall but rather flexed using one or more of the following options: a wall that comes up to the ceiling; a sliding wall or door; or a freestanding bookshelf unit. Since many of these options are manufactured and installed by the same companies who once specialized in pressurized walls, the end result is often similar but with one notable difference—in most cases, these options are technically in compliance with DOB standards. This reflects the somewhat murky way in which “wall” is defined. Technically, if a wall doesn’t reach the ceiling, is not permanently attached, or can pass as furniture—as is true of free-standing bookshelves—your flex is probably legal, even if it has the effect of radically altering the layout and potential use of your apartment.

From the Listing: Sunlit and beautifully renovated convertible two-bedroom home at Riverview East: one of the most desirable doorman buildings in Kips Bay. This east-facing high-floor apartment is flooded with sunlight. It features open city views, a generously proportioned living space, a flexible open layout, and four oversized custom outfitted closets (two of which are walk-ins). See floor plan and full details here.

3. Do I need to get permission from my board, owner or management company?

It is highly advisable that you get the green light from your owner and/or management company prior to flexing your unit. If you rent in a condo or co-op also double-check to ensure your building’s board approves your plan. If your owner, management company, or board doesn’t approve, your potentially costly investment in a partial wall, sliding wall, or freestanding bookshelf that function as a wall may be lost. Also, it is important to note that most reputable companies that build and install temporary and sliding walls or freestanding bookshelf dividers will ask you to get approval from the unit’s owner and/or building management prior to doing any work in your unit.

4. Am I responsible for removing the wall when I move?

This is a tricky question. If the incoming tenants are excited about the additional wall and the owners have, in fact, benefited from the ability to rent out your apartment as an already legally flexed unit, you’ll probably not experience any problems. However, since most leases contain a clause requiring you to leave the unit in the same conditions you rented it in, if you fail to remove a partial wall, sliding wall, or freestanding bookshelf-divider you could be held responsible and as a result, lose your security deposit. As a rule, this is something you should also discuss with your owner or management company at the time you ask for permission to do the installation.

5. What is the typical cost of flexing and what’s the potential return on investment?

Partial walls and freestanding bookshelf dividers often start at just over $1,000 but bear in mind that you get what you pay for. In other words, pricier options generally look less like temporary walls and more like permanent walls. Also, as you might expect, the larger the wall and the more features—for example, the integration of built-in bookshelves, doors, sliding doors, or windows—all add to the price. If you want to opt for a sliding door from a company like Raydoor or a custom-designed Japanese sliding wall from the master builders at Miya Shoji, you’ll likely pay at least $4,000..

Whether you’ll ever see a return on investment depends on how long you stay in your flexed unit. If, for example, you spend $5,000 to legally flex a $3,000 one-bedroom apartment into a two-bedroom apartment in a neighborhood where two-bedrooms generally start at $3,500, over five years, your initial $5,000 investment could result in a savings of $25,000 or more.

From the Listing: Sun-flooded and pindrop quiet convertible 2 bedroom home in a full-service luxury cooperative in a prime location within Sutton Place! The oversized living room features two cheerful exposures (north and east) providing lots of natural light, a dining/work-from-home area that easily converts to a second bedroom. See floor plan and full details here.

From the Listing: Best Value Convertible Two Bedroom. Approximately 900 SF one bedroom apartment with a separate dining area, easily convertible into a second bedroom or office. See floor plan and full details here.

From the Listing: Pin-Drop Quiet Junior 4/Convertible 2 Bedroom in Prime UES. Nestled on a quiet residential street in the Upper East Side, this north-facing, convertible two-bedroom apartment at 345 East 73rd Street is sure to impress with its Brazilian Cherry Hardwood floors, recent renovations including Porcelanosa tile bathroom, and central location. See floor plan and full details here.

From the Listing: Welcome to 3-K, a renovated convertible two-bedroom/ one bath condominium in the Seaport South Financial District. This home has beautiful new wood floors throughout, offers a large entry foyer with two walk-in closets, open living/dining/entertaining space, a custom built banquette with hidden storage and is currently configured with a home office/nursery. The master bedroom is generously sized complete with a dressing area and more generous closet space. 3-K has six closets in all! See floor plan and full details here.

From the Listing: This very spacious 1-BR / convertible 2-BR has been updated with light, inviting finishes, and features a pass-through kitchen with stainless steel appliances, a library nook, and 2 large hall closets in the living space. The king-sized bedroom has an impressive walk-in closet and space perfect for a sitting area or an in-home office space. There is even a rare treat of a dressing area between the bedroom and the en-suite bathroom (positioned for easy, non-intrusive access for guest use as well). Don't miss out on the opportunity to own a convertible 2-BR home in the luxurious 60 Sutton Place South. This home is located in a full-service white glove building with 24 hr doorman, concierge, fitness center, valet service and discounted on-site parking. Pied-a-terre and pets are welcome. See floor plan and full details here.

From the Listing: Spacious one bedroom, flex two-bedroom features approximately 800 sqft of living space and Amazing light/Views from double exposure Over-Sized pivoting windows. Enjoy wonderful protected views of the Empire State building and iconic NYC buildings. This bright North & West facing apartment overlooks a beautiful city park that protects your view forever. The galley kitchen offers Stainless steel appliances and granite countertops. Beautiful mint condition mahogany hardwood floors run throughout the apartment. The unit features great closet space with 2 large double closets plus a large linen closet. The expansive floor plan has amazing potential for open kitchen and great room concept and/or adding a 2nd small bedroom or office. See floor plan and full details here.

From the Listing: Move right in to this newly renovated, bright, oversized one-bedroom corner unit at The Hamilton one of the most coveted full-service buildings in the heart of Greenwich Village. This well-proportioned home features a grand foyer, generous living room and an exceptional row of south-facing windows that provide golden light and treetop views throughout. See floor plan and full details here.

From the Listing: Large convertible 2 bedroom, 1 bath condo with terrace and excellent closet space. Washer/dryer permitted! Garage in building! Nicely renovated with open kitchen layout. Located at the prestigious Carriage House Condominium on the Upper East Side. See floor plan and full details here.

From the Listing: Spacious 1100sf loft with tall ceilings and a 50 foot great room. This convertible two bedroom apartment has ample room for an additional home office area. Pin drop quiet, renovated kitchen, and new central air conditioning system. 176 Broadway is a prewar limestone building with landscaped roofdeck, new elevators, renovated hallways, laundry on every floor, storage, bike room, live-in super and part-time doorman. The lobby is also being renovated. Monthly maintenance includes heat and electricity. Pet friendly. Some images are virtually staged. At less than $800 per square foot, this is the best priced downtown loft in Manhattan. See floor plan and full details here.

From the Listing: Steps from the Seaport, Brookfield Place, and Wall St, this spacious, convertible 2 bed/1.5 bath luxury unit located at 59 John Street features every detail you can possibly think of, including brand new floor to ceiling sliding glass doors for the office or 2nd bedroom. Included with the purchase price is a large, deeded PRIVATE STORAGE unit. See floor plan and full details here.

From the Listing: WORK AT HOME! FABULOUS BALCONY! No detail was overlooked in this lovely convertible two-bedroom home's meticulous renovation. Enjoy the sunny, expansive living room flanked by an oversized west-facing balcony offering wide-open sky views and plenty of room for al fresco dining. See floor plan and full details here.

From the Listing: Welcome to #3N at 382 Central Park West! Move right into this highly desirable 1-bedroom, convertible 2-bedroom home. This spacious apartment spans 840 square feet, has a large private balcony and is located just half a block away from Central Park. See floor plan and full details here.

From the Listing: Welcome to this rarely available one bedroom/convertible 2 with 1.5 baths in an iconic Rosario Candela building on the Gold Coast of lower Fifth Avenue. See floor plan and full details here.

From the Listing: Staggeringly Great Location - Fabulous and Flexible One Bedroom/Convertible Two Close to Village and SoHo. See floor plan and full details here.

From the Listing: Newly Available! Light-filled open concept apartment right next to Madison square park with designer finishes and light-filled living space. This convertible two-bedroom, two-bathroom residence is a rare find in a full-service Flatiron condop. See floor plan and full details here.