Bolster combined a two-bedroom, two-bath apartment with an estate condition studio in an Art Deco building in Union Square.


How much does it cost to combine apartments in New York City?

If you (like every New Yorker) need more space, an apartment combination may be the answer—and it’s more accessible than you might think. 

In terms of the actual renovation work, combining co-op or condo apartments usually just requires removing a kitchen and creating a doorway-sized opening in a wall to join the two units. 

“You could get that done for around $5,000 to $10,000—but to create something that will be worth more than the sum of its parts on resale and actually feels like a home in terms of flow, functionality and durability, most people undertake pretty extensive renovations,” says Fraser Patterson, a former general contractor and the founder of Bolster, a New York City based firm that has designed a relatively seamless renovation experience.

Unlike other design build firms, Bolster provides homeowners with radical transparency and a zero financial risk guarantee. Through the use of a proprietary data-driven estimating tool, Bolster is able to provide homeowners with a more accurate estimate of overall costs. This is no small feat, given that half of the renovation projects in the U.S. go 40 percent to 200 percent over budget. The firm also boasts a vertically integrated services structure that allows for a full team approach to renovation; architect, designer, contractor and builder work together from inception to completion of the homeowner’s project

A typical apartment combination might entail a gut renovation to enhance the layout of the finished home, de-commissioning kitchens (most NYC apartments can only legally have one; more on that below), and moving waste and mechanical systems.


While gut renovations in New York City typically run anywhere from $216 to $526 per square foot depending on the level of luxury and craftsmanship involved, there are some other variables to consider when combining apartments that can drive the project to the higher end of that range.

Based on the data Bolster has collected about this type of renovation in NYC, Fraser says the average cost per square foot is $388, with projects taking an average of five to eight months in the design phase, and six and a half to seven months in the build phase.

If you’re considering an apartment combination, here’s what you need to know.

Co-op or condo? Do your homework with the building 

Before you begin, there are some measures you can take to help move your project along smoothly.

“If you’re not on the board of your co-op or condo, join it now,” says Anna Karp, co-founder of Bolster. “It’s a way to learn the workings of your building, and can be helpful for keeping the approval process moving along efficiently.”

Bolster architect Michael Fasulo, AIA, recommends confirming with your building management that they will allow a combination, as some boards are lenient while others are very restrictive. Co-op combinations typically require an Alteration Type II application. Condos require a special filing with the Department of Finance to amend the tax lot. 

The alteration agreements for condos sometimes have special caveats that allow renovators to own the hallway. 

“It's important to look into the bylaws when doing a combination, which may benefit from ‘eating up’ a bit of the hallway,” says Anna. 

Michael also recommends making sure you do not need to amend the Certificate of Occupancy. Generally this won’t be necessary unless you are connecting a different use group to your apartment (for example, annexing a doctor’s office to an apartment).


Get a smart financial start

One way to help mitigate costs is by engaging the right professionals.

“When a project is planned appropriately from the get-go, the areas where there may be a price increase are highlighted by the team. These include, for example asbestos, structural engineering or electrical updating,” says Anna.

For vertical combinations (more on those below), it is recommended that you engage a structural engineer early on in order to assess the viability of the project. For example, buildings constructed in the 1970scan present more challenges due to the building materials used for construction then, such as solid concrete. 

Another option to help mitigate costs, if you’ve already purchased an apartment adjacent to your own for a combination, is to rent it out before work begins. This can be done by including a “get out clause” in your lease, which stipulates that you can ask the tenant to leave before a year is up, according to Anna.

A lease must have very specific language about renovations, so do your due diligence before crafting such a clause. If done correctly, it could be a smart way to alleviate some costs while you plan in the event that you get the approvals you need and are ready to start renovating.

“It takes a lot of pressure off the process,” Anna says. “You’ll have time to think about design and get board and DOB approvals without losing a lot capital.”

Another potential money-saver: “If you’re not completely gutting both apartments, you may be able to jump from one place to the next,” says Fraser. Ask your contractor or architect during the planning phase whether it’s feasible for you to remain in your home while work is underway rather than renting elsewhere temporarily.

The layout of the combination makes a big difference

Once you’re ready to begin work, there are multiple factors that can influence budget and timelines. 

For example, a duplex combination will be much more expensive than a so-called "adjacent" apartment combination, where the apartments are side-by-side on the same floor.

“Side-by-side renovations can be as simple as popping a hole in the wall and taking out the stove, but if you want it to feel natural, it often requires substantial changes to both units,” says Michael.

Vertical combinations are more complicated require extra thought, as you will likely need to do serious structural work, such as building a staircase or adding or restructuring beams. The project will also likely necessitate demolishing floors, as well as putting in new steel, which homeowners will likely need to hire a structural engineer to handle. 

“A vertical combination adds negotiation time with your building, planning time, cost for steel, fireproofing, and inspections. Plus, you’re going to want those stairs to look amazing after going to all that trouble, so you’ll probably want to spend a little there, as well,” says Michael. 

All told, a vertical combination can add another $30,000 to $100,000 to your project.


Keep an eye out for mismatches in quality and finishes

Combining apartments that are mismatched in quality—that is, if one has been renovated and one hasn’t—will pose additional design challenges. You’ll want to match the level of finish in the apartments you’re combining so that you don’t feel like you’re walking from a beautiful renovation into a mediocre one.

“The sad truth here is that if they are seriously mismatched, they both usually get mostly gutted to make them work together,” says Michael. “If you’re being frugal and want to preserve one unit, use the newer one as the design baseline. Try to matching the doors, trims, and other details.” 

Anna recommends having a design vision in mind, and asking the building for help. 

One way to ensure that your completed combination has a seamless feel is to ask building management to share with you the specifications of the original apartments, as well as any prior renovations to the space, in order to better match the flooring, finishes, light fixtures, and other details. 

“Sometimes, in condos that were developed recently, it's a good idea to approach the board to see if there are floorboards left over, or any other fixtures that may have been purchased wholesale and that would help with matching,” Anna says.

You may also encounter another mismatch in the levels of your spaces. Art Deco apartments, for instance, frequently feature sunken living rooms. If your living room goes down, remember that means that the apartment of your neighbor above also goes down, so combinations in these instances will involve matching ceiling planes. 

Know the rules for kitchens and bedrooms

As mentioned above, New Yorkers are only permitted one kitchen per apartment—although an exception may be made you have a letter from a rabbi saying that you need a separate, kosher kitchen. A kitchen demolition runs anywhere from $5,000 to $8,000. On the bright side, this can present exciting opportunities to change the flow of space and consider new floor plans. 

Note that there are also legal requirements for creating an extra bedroom. There are minimum size requirements (typically 8 by 8 by 8 feet) as well as rules for natural light and ventilation (you can’t build windowless bedrooms, for example).

Gas and plumbing lines could present challenges

The challenges for an apartment combination from this perspective are the same as a gut renovation, except when there are major variances in the condition of the systems (for example, if one of the apartments is in estate condition).  

“These problems are standard for any renovation in NYC. In vertical renovations they can be a challenge when these systems pass through the area of the stair and need to be rerouted,” says Michael.

Plumbing risers are the most expensive thing to move around. If one apartment has a bathroom that lines up to the other apartment’s kitchen, you won’t necessarily be able to blast through the wall to create a giant kitchen or bathroom.  The risers—which begin in the basement and end at the top floor—may serve a whole line of apartments, so building managers often forbid moving them for safety reasons and because in past renovations, they’ve been moved improperly in a way that’s caused problems in the building. If you are allowed to shift the riser line, it will cost around $10,000 to $15,000.  

Consider installing an HVAC system

If your building allows it, you will probably want to install an HVAC system for central air conditioning. At the new price point your combined apartments will command, it’s an amenity future buyers may expect as they compare your home to similarly priced but more modern condominiums. Expect to pay around $40,000 to $80,000 excluding carpentry expenses to make the HVAC system disappear into the aesthetics of your apartment. While some air conditioning companies provide an in-house engineer to install it, it’s recommended that you hire a mechanical engineer for the job to ensure it’s well-integrated and takes up less space. 

The ins and outs of new windows

Replacing windows is fairly easy, costing around $1,500 to $3,000 per window. If your apartment is in a landmark building, you may need to ensure that your windows conform to landmark preservation rules. (Windows on a public façademust conform to a landmarks-approved window replacement scheme. If it doesn’t, your window supplier will have to submit an application to Landmarks showing how the new window design closely reflects the existing conditions.) Creating a new window opening in a wall is often an option, too, in older buildings that have side or rear exposure to take advantage of. You’ll need a skilled architect to help win board approval for this; expect to pay around $3,000 for the new opening.

Millwork may be your largest expense 

You’ll want to make sure the apartments blend seamlessly, which almost always requires updates to trim, moldings, doors and windows. That, along with cabinetry including any built-ins, will probably be the largest expense in a gut renovation budget, and can vary significantly depending on the level of quality you’re seeking. Custom or semi-custom millwork requires skilled labor, and can get quite pricey—but such details can make a space feel much larger. For high-end custom millwork assuming full height bookcases, storage, cabinets or elaborate wall panels, prices can range from $200 per linear foot to $800 per linear foot. This ranges enormously depending on the type of wood and veneer and how elaborate the interiors are divided and broken up into drawers, shelves, etc.

Architect fees

Apartment combinations nearly always require an architect—an experienced one who will be able to handle the whole scope of the project, from bidding through overseeing the job’s management. Expect to pay anywhere from 10 to 20 percent of your project’s total cost (including labor, materials and appliances) depending on your architect’s skill and level of experience. (Note: A good architect can often integrate interior design for a more efficient process and synthesized design.) 

Extra costs to look out for

Unexpected little extras can add up, so here are a few to look out for: You will need an expeditor ($1,000 to $3,000 depending upon level of service) to help navigate city bureaucracy. And along those lines, you will also have to pay fees for city permits and asbestos inspections; the latter may require a third party monitor, if a significant amount of asbestos is found.

There will probably also be a number of payments to make to your building, for protection of hallways, doors, and elevators ($800 is not unusual), for any water or gas shut offs a renovation requires (some buildings charge $500 per shutoff, while others charge nothing), a general construction charge to the building (anywhere from $2,000-$6,000 per month or a percentage of your total construction costs), and penalties for going over the scheduled renovation time ($50-$200 or more per day).

Then there are the change orders—last-minute changes to the project can add up to 10 percent of its total cost—but these 11thhour alterations are almost always initiated by owners, says Fraser. While many people recommend setting aside 20 percent extra to include "surprises," Fraser maintains that this shouldn’t happen if a competent architect and general contractor are involved (and engaged/paid) who do adequate due diligence beforehand.

Is an apartment combination right for you? 

New Yorkers know how to make the most of small spaces. But when being well-organized and resourceful isn’t enough, an apartment combination may be the answer. 

“Combining can be a smart investment decision, as the property as a whole may be worth more than its parts,” says Anna. 

The Bolster Smart Renovation Zero-Risk Guarantee

How can a design-build firm guarantee a Zero-Risk renovation?

Bolster has pioneered Smart Renovation. We apply quantitative analysis along with our proprietary technology solution to identify and quantify the performance risk on every renovation project. The result is a personalized strategic approach to each renovation that allows us to absorb 100 percent of the homeowner’s risk. Your home will be beautifully designed, and delivered on-time and on-budget. That is our guarantee.

Smart Renovation & Zero-Risk means that Homeowners are now free to dream.

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