Build Double Story Flat Roof Extension

Can you really add a double storey flat roof extension to your Brooklyn home-two full new levels-without blowing up your budget, your light, or your relationship with the DOB and your neighbors? On a tight Park Slope brownstone three years ago, we added a full-width ground-plus-first extension under a terraced flat roof, and the finished project looks like it was always part of the building-no zoning fights, no neighbor lawsuits, no leaks.

But you don’t get that result by starting with Pinterest floor plans and asking a contractor for a price. A double storey flat roof extension changes how your property stands, drains, and occupies the sky-so you need to design from the outside in, starting with structure, waterproofing, height limits, and neighbors, then working into room layouts and window sizes.

What a Double Storey Flat Roof Extension Really Means

A double storey extension isn’t just a bigger bump-out. You’re stacking two full floors of new construction-typically ground-to-first or first-to-second-under one or more flat roofs.

Two Levels of New Structure

Each new level carries live loads (people, furniture) and dead loads (walls, floor, and roof assembly). That means you need proper foundations, vertical supports down to grade, and structural floors and roofs at each level. You can’t lightly attach two new storeys to the back of an old brownstone and hope the existing walls hold it all up.

Flat Roofs as Both Lid and Platform

Your uppermost flat roof might be a simple weather lid. Or it might become a terrace, green roof, or mechanical platform. The decision affects structure, waterproofing, and safety rails from day one. If you think “we’ll figure that out later,” you’ll be opening up the roof assembly to add structural support and guardrail blocking in a few years.

Integration with the Existing Building

New floors have to line up with existing floor levels. New stairs have to connect circulation smoothly. New window sills and headers should relate to old windows so the facade doesn’t look chaotic. And externally, your new mass-two storeys taller than the yard-needs to sit comfortably beside party walls and in front of neighbors’ rear windows.

Layout Strategies for a Double Storey Flat Roof Extension

There’s no single “correct” plan for a double storey flat roof extension. The strategy you pick shapes cost, light, drainage, and neighbor impact. Here are three common approaches I’ve used on Brooklyn projects.

Full-Width Rear Stack

Both new levels run the entire width of your lot at the rear, aligning with party walls on both sides. Common pattern: big open kitchen-living-dining at the lower extension level, bedrooms or offices above, under one continuous flat roof. Structure is straightforward (load walls at party lines), and drainage can run to one or two internal drains near the center. The challenge? A full two-storey rear wall can dominate neighbors’ views and block afternoon light into their yards. You mitigate this by stepping the upper storey back or carefully placing windows to avoid direct sightlines.

Example: On a narrow Clinton Hill brownstone we handled the rear extension this way-full-width lower floor for a kitchen opening to the garden, upper floor with two bedrooms and a shared bath. The flat roof above is a simple membrane with a low parapet; no terrace was needed, so we saved structure and kept the height modest.

Side-Offset or L-Shaped Extension

The new volume might occupy one side of the yard more deeply, leaving a courtyard or side light slot along the other edge. Internally, this brings daylight into the middle of the house through the open side. Externally, you create inside corners where two extension walls meet-these require careful flat roof drainage detailing so water doesn’t pond at the corner. The advantage is less perceived bulk and better daylighting; the cost is more complex waterproofing and potentially more expensive framing at the “L.”

Stepped Heights Between Storeys

The upper storey steps back from the lower one, leaving a roof terrace or garden over the first extension level. This reduces bulk as seen from neighbors’ windows and creates a private outdoor room at mid-level. Each step introduces extra parapet edges, flashing lines, guardrail bases, and potential for trapped water if not designed carefully. But the trade-off-better neighbor relations and usable outdoor space-often justifies the extra detailing.

Structure and Foundations for Two Storeys Under a Flat Roof

Adding two levels changes how loads travel to the ground. You can’t treat a double storey extension like a simple deck tacked onto the back.

Carrying Two New Floors and a Roof

Each floor adds roughly 50-70 pounds per square foot (psf) dead load plus 40 psf residential live load, depending on joist depth and finishes. The flat roof adds another 20-30 psf dead load, plus snow load (25 psf for Brooklyn) and any terrace or green roof use (60-150 psf extra if the roof is occupied). Engineers will calculate the total and design new foundations-typically piers or continuous footings-to handle it all without overloading existing brownstone footings.

Tying New Structure into Old

Your new extension walls or columns need to bear on new footings, not just rest on the existing rear wall footing. At each floor level, new joists or beams may bear into pockets in existing masonry party walls (which triggers a party wall agreement with neighbors), sit on new steel frames parallel to the old walls, or land on posts inside the extension footprint. The structural engineer will check the existing walls’ capacity and recommend reinforcing or underpinning if needed.

Span and Column Strategy

Decide early where columns or bearing walls can go inside the new levels. Large open rooms mean longer joist spans, deeper beams, and more expensive steel or engineered lumber. If you can accept a post or thickened wall in the middle of the lower-level kitchen or a bearing wall between upper-level rooms, you’ll reduce structure costs and keep joist depths manageable-which in turn means thinner floor builds and more ceiling height.

Flat Roof Build-Up for a Double Storey Extension

You’re creating at least one new flat roof, possibly two if the lower extension level gets its own roof plane. Each must drain reliably and stay dry in Brooklyn’s freeze-thaw cycles and summer storms.

Warm Roof Over the Upper Storey

The top roof assembly typically looks like this: structural deck (plywood or concrete over joists) → air and vapor control layer → rigid insulation boards (often tapered rigid foam to create slope to drains) → cover board (dens deck or similar) → waterproofing membrane (mod-bit torch-down, TPO single-ply, or liquid-applied). This keeps the upper ceiling warm and the roof structure above the dew point, minimizing condensation inside the assembly.

Intermediate Roofs and Terraces

If your first extension storey has a roof that’s used as a terrace or left open to the sky below the second storey, that roof also needs full waterproofing-usually an inverted or protected membrane roof (PMR) if it’s a terrace. In a PMR, the membrane goes directly over the deck and vapor control, then rigid insulation sits *on top* of the membrane, protected by filter fabric and pavers or ballast. This protects the membrane from UV, foot traffic, and freeze-thaw. Guardrails, planters, and any bulkheads or stair enclosures at this level must be anchored through the waterproofing with proper flashing boots or blocking.

Drainage Over Multiple Levels

Each roof plane needs clear drainage to internal drains or scuppers; water shouldn’t have to travel across an entire two-storey extension to exit at one corner. On a stepped extension, the upper roof drains independently, and the lower terrace roof has its own drains-often both tying into the same vertical leader inside the extension wall, but each entering at its own level with overflow scuppers as backup. Missing or undersized overflow routes are one of the most common flat-roof failures I see on multi-level projects.

Walls, Windows, and Thermal Performance in a Two-Storey Extension

The vertical envelope wraps both new storeys and meets the flat roof at the parapet. Get this wrong and you’ll have leaks, cold spots, and high energy bills across two floors of new space.

Continuous Insulation and Air Barriers

Insulation should run continuously from foundation to roof without major thermal bridges at floor lines, beams, or balcony edges. The air barrier-often the vapor control layer on the warm side of the insulation-must also be continuous, taping and sealing at every joint, penetration, and transition from old to new construction. On double storey extensions I’ve designed, we typically use a fluid-applied air barrier on the exterior sheathing that laps into the existing building envelope at the rear wall and continues up to the roof parapet coping, avoiding leaky joints at the new floors.

Glazing at Both Levels

Large openings at the lower level connect to the yard and flood the kitchen or living space with light. Upper-level glazing should be sized and placed to capture sky views, align with existing window patterns for a calm facade, and avoid overheating the bedrooms. Too much glass without shading or proper orientation means summer AC loads and winter heat loss. Use high-performance double-glazed units (U-factor ≤ 0.30) and consider exterior shades or deep reveals at south-facing glass.

Fire Separation and Egress

If your extension creates new sleeping rooms on the upper level, code requires egress windows or a secondary exit route. Party walls between your unit and neighbors must meet fire-resistance ratings (typically one-hour), and the floor between your new storeys may also need fire protection depending on use. These requirements affect joist sizing, drywall detailing, and where you can put doors or windows near lot lines. And if the new flat roof is a terrace, you’ll need code-compliant guardrails (42″ high in NYC) and possibly a fire-rated stair or bulkhead enclosure if the terrace is used as an egress path.

Planning, Zoning, and Neighbor Relations in Brooklyn

Before you sketch a single floor plan, check your zoning envelope and talk to your neighbors. Brooklyn’s zoning rules and social expectations shape what you can actually build.

Height, Bulk, and Rear Yard Limits

Zoning controls total building height (often 30-40 feet in R6 or R7 residential zones), number of storeys, and how far into the required rear yard you can extend. A double storey extension will often push close to the maximum. Your architect will calculate floor area ratio (FAR)-total building floor area divided by lot area-to confirm you’re not over the allowed density. Rear yard rules typically require a percentage of the lot depth to remain open; extensions must fit within the “buildable” zone. Get a zoning analysis done *first*, before you invest in detailed design.

Light, Privacy, and Views

Two new levels can block afternoon sun or views into neighbors’ yards. Stepping back the upper storey, using clerestory windows high on rear walls, and carefully orienting windows to avoid direct sightlines into neighbors’ windows all help. Where the new flat roof becomes a terrace, consider what neighbors see when you’re standing on it-can they see into their bedrooms or yards? Privacy screens, planters, or strategic guardrail design (frosted glass panels instead of clear cable rail) can soften the impact, but they must attach to the roof without compromising waterproofing.

Landmarks and Co-op/Condo Rules

In landmark districts (Fort Greene, Brooklyn Heights, Park Slope historic zones), rear and rooftop changes can still come under Landmarks Preservation Commission review. LPC will scrutinize visible flat roof parapets, railings, and bulkheads from public streets and neighboring properties. Use materials and profiles that read “background”-dark metal rails, low parapets, minimal rooftop equipment visibility. In co-op or condo buildings, boards may have their own design review and use restrictions for extensions and new roof terraces; get board pre-approval before you file with DOB.

Using the New Flat Roof: Deck, Garden, or Services?

What you plan to put on top of your double storey extension drives structural, waterproofing, and safety design from the start.

Amenity Roof Terrace

Design for pavers on adjustable pedestals or hardwood decking on sleepers, both sitting above a protected membrane. Structure must support 60 psf live load (code minimum for assembly occupancy) plus dead load of pavers, pedestals, and any furniture or planters. Guardrails need blocking or curbs anchored through the roof assembly with proper flashing boots. Access via a stair bulkhead or large roof hatch. Include lighting, drainage under the paver grid, and irrigation rough-in if you want container gardens.

Green Roof Over Extension

Extensive green roofs (shallow soil, sedums) add 15-25 psf saturated weight; intensive green roofs (deeper soil, shrubs) can add 50-150 psf. Membranes must be root-resistant (reinforced TPO or EPDM with root barrier) and protected with a drainage mat and filter fabric. Overflow routes are critical-soil and plants will slow drainage, so emergency scuppers must handle extreme rainfall. Plan for maintenance access and irrigation if you want more than a self-sufficient sedum mat.

Mechanical and Solar Platforms

Dedicating part of the new flat roof to condensers, ERV/HRV units, and solar panels keeps noisy equipment away from windows and yards. Equipment sits on curbs or vibration-isolating pads; panel arrays mount on ballasted or attached racking systems. Cable paths, condenser drain lines, and refrigerant lines must be coordinated with the waterproofing-every penetration is a potential leak if not detailed with proper flashing collars. Combine this with a terrace zone by screening equipment with low planters or metal panels that also serve as guardrails.

Phasing and Budget Strategy for a Two-Storey Flat Roof Extension

A double storey flat roof extension in Brooklyn typically costs $400-$650 per square foot of new interior space, depending on finishes, glazing area, and roof use. On a 400-square-foot footprint (two storeys = 800 square feet new), expect $320,000-$520,000 all-in. Structure and exterior envelope (framing, roofing, windows, waterproofing) usually represent 50-60% of that total.

Structure and Shell First

Phase 1 covers foundations, framing, exterior walls, flat roofing, and windows to a watertight shell. This is non-negotiable-you can’t pause mid-roof or leave structure exposed over winter. Getting the envelope right means the rest of the project (interior fit-out) can proceed without weather risk or leaks damaging new work.

Fit-Out and Services on Each Level

Once the shell is weathertight, you can phase interior finishes if budget is tight. Finish the more critical level first-typically the lower kitchen-living space-and the upper bedrooms later. But rough in all mechanical, electrical, and plumbing (MEP) for both levels in Phase 1, including drain and vent stacks, HVAC distribution, and electrical panels, so you don’t have to tear into finished walls or ceilings down the road.

Roof Amenities as a Subsequent Phase

If you can’t afford a full terrace or green roof build-out straight away, design the roof assembly and structure *now* to support it later. Include extra live load capacity, blocking for future guardrail posts, and a protected membrane with drain access-even if you delay pavers, decking, or plantings by a year or two. This avoids opening up the roof assembly for structural retrofits once the project is done.

The Team You Need for a Double Storey Flat Roof Extension

You can’t hand this project to a single “super-contractor” and expect good results. A double storey extension requires coordinated design and specialized execution across multiple disciplines.

Architect

Leads design, massing, layouts, and facades. Interprets Brooklyn zoning, landmark requirements, and coordinates structural and roofing strategies. Files with DOB and represents the project through plan exam. Expect to pay 10-15% of construction cost for full architectural services (schematic design through construction administration).

Structural Engineer

Designs foundations, beams, joists, and connections for two new storeys and all roof loads. Checks existing walls and framing for added loads and recommends reinforcements or underpinning. Reviews shop drawings for steel and engineered lumber. Structural engineering fees typically run $8,000-$15,000 for a residential double storey extension.

Roofing Specialist

Advises on roof assemblies, drainage, and flashing details at parapets, terraces, and penetrations. Executes or oversees membrane and insulation installation, especially at junctions with existing roof areas. For a complex multi-level flat roof, use a roofer with commercial or high-performance residential experience, not a shingle-only crew. Roofing cost for both levels might be $35,000-$60,000, depending on membrane choice, terrace build-up, and guardrail integration.

General Contractor / Builder

Coordinates all trades, schedules DOB inspections, manages site logistics on a tight Brooklyn lot, and ensures temporary protection for neighbors and existing interiors during demolition and construction. Choose a GC with double storey or multi-level addition experience-the sequencing and shoring are more complex than single-storey work.

What to Decide Before You Approach a Designer or Roofer

Clarify these points before your first meeting with an architect, engineer, or roofing specialist. Clear answers get you better proposals and realistic budgets.

• Which floors you want to extend (ground + first, first + second, etc.) and roughly how much new area on each level (200 sf, 400 sf, full lot width?)
• Whether the new flat roof should be a terrace, green roof, mechanical platform, or simple weatherproof lid-and if you’re unsure, whether you want the structure sized now for future terrace use
• Your must-have rooms or functions in the new levels (larger kitchen, extra bedroom, home office, rental unit, all of the above?)
• Any constraints you already know: landmark status, co-op/condo board rules, difficult neighbors, or limits on yard loss
• Your general budget range and tolerance for phasing (core structure and roof now, finishes or terrace amenities later)

Turn a Double Storey Flat Roof Extension from Idea to Buildable Plan

Two new levels plus a flat roof can completely reshape how a Brooklyn property works-doubling your living space, adding light, and creating a private rooftop in the middle of the city. But only if you treat structure, envelope, and planning with the same care you’d give to interior finishes.

A well-designed double storey extension gives you light, space, and a usable roof for decades. A rushed one locks in leaks, neighbor conflicts, and structural problems just as long.

Start by gathering photos of your existing building, a survey or tax map, and a written list of your goals and constraints. Reach out to a Brooklyn-based architect, structural engineer, and roofing specialist-ideally ones who’ve worked together on similar projects. Ask them to sketch two or three massing and roof-use options, with rough phasing and budget outlines, so you can choose a path that fits your ambitions, your lot, and your reality. Then design from the outside in-load paths, waterproofing, drainage, and height limits first-so the clean two-storey box you see in the renderings can actually stand, stay dry, and get approved in real Brooklyn conditions.