Multi-Unit Apartment Renovation: Combining Spaces and Blending Interior Design Styles

Renovating an apartment by combining multiple adjacent units into one home presents distinct design challenges that go beyond simple demolition and reconstruction. The structural layout of the original building, the placement of existing plumbing and electrical risers, and the need to create a cohesive aesthetic from multiple pre-existing spaces all demand careful planning. When a couple with two separate households and different design sensibilities decides to merge their homes, the complexity increases further. These large-scale residential renovations require expertise in structural engineering, interior design, and building code navigation. The same principles of thoughtful integration apply whether combining two studios or three full apartments, and lessons from New York City projects align closely with passive house design approaches in New York City where energy efficiency and spatial efficiency go hand in hand.

Combining Multiple Apartments into One Cohesive Unit

Combining apartments – often called a horizontal merger in real estate terms – involves removing walls between adjacent units, reconfiguring existing layouts, and integrating mechanical systems that were designed for separate dwellings. In post-war co-op buildings, the structural system typically consists of concrete slabs and columns, which means interior partition walls are generally non-load-bearing and can be removed with minimal structural reinforcement.

Structural Assessment Before Combining

Before any walls come down, a structural engineer must verify which partitions are load-bearing. In concrete-framed buildings, the columns and slab edges carry the structure, so most interior walls are infill partitions. Key steps in the pre-construction assessment include:

1. Review original building plans to identify column locations, slab thickness, and the structural grid.
2. Core test the concrete slab at proposed wall removal locations to verify reinforcement layout.
3. Locate all plumbing, electrical, and HVAC risers within the walls to plan relocation or extension.
4. Check co-op board alteration agreements for restrictions on unit mergers – some buildings limit the number of units that can be combined on a single floor.

Pre-war penthouse design with urban greenhouse integration demonstrates how even historic building constraints can be overcome with creative structural solutions, particularly when combining existing units demands careful load path analysis and coordination with landmark preservation requirements.

Mechanical System Integration

Each original apartment had its own heating riser, electrical panel, and sometimes its own air handling unit. Combining units means consolidating these into one system per merged unit. Electrical panels can be combined at the meter – or sub-metered if the co-op requires separate tenant metering. Through-wall HVAC sleeves must be re-evaluated because the merged space may have different heating and cooling loads than the sum of its parts, especially when interior walls are removed and open-plan zones are created.

Merging Two Distinct Interior Design Aesthetics

When two people with established homes and different aesthetic preferences create a shared living space, the interior design process must bridge two visual languages without diluting either. The neutral palette strategy – using soft greens, off-whites, pale yellows, and taupes as a backdrop – allows each person’s furniture and art collection to stand out without clashing against competing wall colors or patterns.

Strategies for Blending Two Design Styles

• Start with a neutral base – walls, flooring, and major fixed elements in soft, tonal colors that work with both collections.
• Rotate featured pieces – each zone of the home can highlight one person’s aesthetic while the other’s pieces appear in adjacent zones.
• Use art as the unifying element – when both partners own significant artwork, let the collection define the color direction of each room.
• Invest in custom storage – built-in cabinetry in a consistent finish hides the visual clutter of two households worth of objects.

The architectural community has increasingly addressed how design decisions affect the social fabric of cities. AIA New York has urged architects to stop designing harmful spaces of incarceration, reflecting a broader conversation about professional responsibility in shaping the built environment. Residential designers, by contrast, focus on creating spaces that support wellbeing, connection, and individual expression – goals that align with the careful blending of two personal aesthetics into one harmonious home.

Material Continuity Across Spaces

Using the same flooring material throughout the combined apartment creates visual cohesion. Wide-plank white oak flooring in a matte finish works with both traditional and contemporary furniture styles. Consistent baseboard and door trim profiles throughout the unit – even if the original apartments had different trim styles – unifies the formerly separate spaces into one continuous home.

Working Within Post-War Co-Op Building Constraints

Post-war co-op buildings, built primarily between 1945 and 1970, present specific renovation constraints that differ from both pre-war buildings and new construction. The concrete slab construction typical of this era offers design flexibility because interior walls are non-structural, but the buildings have lower ceiling heights – typically 8 feet, compared to 10 to 12 feet in pre-war buildings – and less robust electrical capacity for modern appliance loads.

Co-Op Board Approval Process

Any renovation that combines units in a co-op building requires board approval, which typically involves submitting detailed architectural drawings, a scope of work, contractor bids, and proof of insurance. The process can take 8 to 16 weeks depending on the board’s meeting schedule and the complexity of the alteration. Key elements that boards scrutinize include:

1. Structural modifications – any column or slab work requires a stamped structural engineering report.
2. Noise mitigation – floor underlayment specifications for impact sound transmission between units.
3. Fire safety – maintenance of existing fire-rated corridors and installation of sprinklers where required.
4. Subcontractor qualifications – licensed, insured tradespeople for all electrical and plumbing work.

Pre-war apartment architecture design lessons from historic New York City buildings offer a useful comparison because the structural constraints differ – pre-war buildings use masonry bearing walls rather than concrete frames, which limits wall removal options but provides higher ceilings and more generous room proportions.

Maximizing Natural Light in Combined Layouts

One of the biggest challenges when combining apartments is that interior spaces that were originally on exterior walls become interior rooms in the merged layout. Natural light must be redistributed through strategic window placement, interior glazing, and reflective surface treatments.

Light Distribution Techniques

• Interior windows and transoms – glass panels above doorways and full interior windows between rooms allow daylight to penetrate deeper into the floor plate.
• Open-plan circulation – reducing the number of enclosed rooms on the interior of the unit allows light from perimeter windows to reach the core.
• Reflective surfaces – glossy tile backsplashes, mirrored cabinet fronts, and light-colored wall finishes bounce available light deeper into the space.
• Glass pocket doors – translucent or clear glass doors provide privacy without blocking light when rooms need to be enclosed.

The call from AIA New York for architects to stop designing prisons and jails reflects a professional movement toward designing spaces that prioritize human dignity, natural light, and connection to the outdoors – principles that directly translate to residential renovations where access to daylight shapes occupant wellbeing.

Displaying Art Collections in Unified Interiors

When a couple with substantial art collections moves into a combined apartment, the art installation strategy can determine the entire interior design direction. Artworks from different periods and artists – ranging from contemporary paintings to vintage photography and sculpture – require thoughtful placement, appropriate lighting, and wall surfaces that neither compete with nor overwhelm the pieces.

Lighting Art in Residential Settings

Track lighting on ceiling-mounted rails offers flexibility for art display because fixtures can be repositioned when the collection rotates. Each artwork should receive 100 to 200 lux measured at the surface, from a 30-degree beam angle for standard pieces or a 10-degree spot angle for smaller works. LED sources with a color rendering index of 90 or above prevent color distortion in the artwork.

Renovating pre-war apartments for modern living presents additional complexities for art display because historic plaster walls may require reinforcement to support heavy framed pieces, and original crown molding proportions limit the usable wall height for large-scale works. Modern resin anchors and french cleat hanging systems solve these structural concerns while preserving historic fabric.

Curating the Visual Flow

A unified home should present the art collection as a curated gallery experience rather than a random assortment. Group works by color temperature, medium, or subject matter along each wall. The formal dining room, for example, benefits from a large-scale cityscape or landscape piece that creates a focal point for dinner guests, while more intimate works suit the hallway or reading nook where viewers can engage with them at close range.

The principle of using a neutral architectural backdrop to feature personal collections – whether art, vintage furniture, or eclectic objects – applies across apartment types and scales. The Classic Six apartment layout in pre-war New York architecture provides a historic precedent for this approach, with formal living and dining rooms designed as flexible spaces that showcase the owner’s furnishings and art within a generous, well-proportioned envelope. Applying these same zoning ideas to a combined post-war apartment – using open living areas as the gallery and enclosed bedrooms as private retreats – produces a home that feels both curated and livable.