What a New Detached ADU Costs and Involves in Westchester
A new detached ADU in Westchester typically runs $90,000-$220,000 and takes 4-7 months from permit approval to final inspection, depending on whether you choose prefab delivery or stick-built construction. Every structure must meet Miami-Dade’s High Velocity Hurricane Zone requirements, including Miami-Dade NOA-rated windows, doors, and roofing. Call (844) 665-0162 and we’ll walk your lot, check setback rules, and give you a written estimate with real numbers.
How We Build a New Detached ADU in Westchester
We’ve learned that Westchester homeowners want the job mapped out before a shovel hits the ground. Here’s what actually happens, and when you’ll need to make decisions.
Stage 1: Site survey and zoning check (1-2 weeks)
We measure your lot, locate the septic or sewer tie-in, and pull Miami-Dade County’s zoning records for your parcel. Westchester sits in unincorporated Miami-Dade, so there’s no city hall to appeal to - county rules govern setbacks, lot coverage, and ADU maximum square footage. You’ll decide at this stage whether the unit is for rental income, family use, or future sale, because that affects parking requirements and utility sizing.
Stage 2: Design and material selection (2-4 weeks)
This is where you choose your build path. We offer prefab units from Mighty Buildings, Boxabl, Plant Prefab, and Tuff Shed alongside traditional stick-built framing. Prefab shortens on-site construction by 6-10 weeks but requires crane access and precise foundation prep. Stick-built gives you full customization on rooflines, ceiling heights, and exterior finishes. You’ll also select flooring, cabinetry, countertop grade, and whether the unit needs impact-rated windows throughout or just the windward elevation.
Stage 3: Permits and engineering (4-8 weeks)
Miami-Dade’s HVHZ permitting is not fast. Structural drawings must be wet-stamped by a Florida-licensed engineer, and every product - from roof shingles to garage door openers - needs a current NOA number. We submit as a package to avoid the back-and-forth that delays so many Westchester projects. During this wait, we order long-lead items like windows and cabinets so they’re ready when the permit releases.
Stage 4: Foundation and rough construction (6-10 weeks)
Westchester’s shallow Miami limestone and near-zero lot elevation mean we often hit rock within 18 inches of grade. Footings may need pinning or a thickened slab design to satisfy flood-zone requirements. We frame, roof, and dry-in, then run MEP rough-ins. At this point you’ll do a walkthrough to confirm outlet locations, switch heights, and any plumbing fixture changes before insulation and drywall close the walls.
Stage 5: Finishes, inspection, and certificate of occupancy (3-5 weeks)
Final inspections in Miami-Dade include wind-load verification, flood-elevation confirmation, and fire separation checks if the ADU is within 10 feet of the main house. We don’t call for final until we’ve tested every outlet, every valve, and every window latch ourselves. The certificate of occupancy is your green light to move in or lease.
The Build Options You’ll Choose Between for a New Detached ADU
Every choice below moves your final number and your long-term satisfaction. We don’t default you to one path.
- Prefab unit (Mighty Buildings, Boxabl, Plant Prefab): Factory-built shell delivered on a flatbed and set by crane. Best for homeowners who want speed and predictable pricing. Trade-off: less flexibility on layout, and you’ll need 14-16 feet of overhead clearance for crane placement. Foundation must be laser-level accurate.
- Stick-built on site: Traditional wood or metal framing built from engineered plans. Best for matching your main home’s roofline, using non-standard dimensions, or siting on irregular lots. Trade-off: longer exposure to weather, more on-site waste, and higher labor hours.
- Tuff Shed pro-series shell with interior finish-out: A middle path - steel-reinforced panelized walls delivered as a kit, then finished inside with standard residential materials. Best for workshop ADUs or budget-conscious rentals. Trade-off: ceiling height and insulation values are fixed by the shell design.
- Slab-on-grade vs. elevated foundation: Slab is cheaper and faster but limits you on flood-zone compliance. Elevated pier or stem-wall construction adds $8,000-$18,000 but satisfies FEMA requirements and gives you covered parking or storage underneath.
- Impact-rated vs. non-impact window package: HVHZ code mandates impact rating or shutters for all glazed openings. Full impact glass adds $3,500-$7,000 over shutters but eliminates the annual install/remove hassle and looks cleaner.
- Standard electrical vs. EV-ready/service-upgrade tie-in: If your main house has a 100-amp panel from the 1960s, the ADU may force a service upgrade. We flag this in week one so you’re not surprised by a $4,000-$8,000 electrical add-on mid-project.
What New Detached ADU Costs in Westchester, FL
New detached ADU construction in Westchester ranges from $90,000 to $220,000. The table below breaks where most projects land based on size and build method.
| Build type & size | Typical range |
|---|---|
| Prefab studio/1-bed, 400-550 sq ft | $90,000-$135,000 |
| Stick-built 1-bed, 500-650 sq ft | $110,000-$165,000 |
| Stick-built 2-bed, 700-900 sq ft | $150,000-$220,000 |
Five variables push you to the low or high end:
- Size and complexity: Every 100 square feet adds roughly $12,000-$18,000 in Westchester’s market. Vaulted ceilings, custom rooflines, or covered porches compound that.
- Foundation conditions: Limestone pinning, flood-zone elevation, or poor soil percolation can add $8,000-$25,000 before walls go up.
- Utility extensions: Tapping into existing sewer and water lines 40 feet from the main house is straightforward. Running new lines 120 feet across a paved driveway, or upgrading from septic, adds $5,000-$15,000.
- Material grade: LVP flooring and solid-surface counters keep costs down. Tile, engineered hardwood, and quartz from Cambria or Caesarstone move you toward the upper range.
- Permit scope and HVHZ compliance: Miami-Dade’s engineering review, NOA product verification, and required third-party inspections add $4,000-$8,000 compared to non-HVHZ jurisdictions. This is non-negotiable and non-negotiable.
Our written estimates include permits, engineering, foundation, structure, MEP rough and finish, insulation, drywall, paint, flooring, cabinets, counters, and fixtures. What typically isn’t included: landscape restoration, fencing, driveway repair from equipment access, furniture, window treatments, and separate meter installation if you want the unit on its own FPL account.
What Westchester Homes Specifically Do to This Job
Westchester’s housing stock shapes every new detached ADU we build here. The bulk of homes are single-story CBS ranch structures from the 1950s through mid-1970s, many with original flat roofs, jalousie windows, and electrical panels that predate modern safety standards. When we site a new detached ADU behind one of these homes, we’re often working around aging infrastructure that wasn’t designed for an additional load.
The High Velocity Hurricane Zone reality hits early and often. Every product needs a Miami-Dade NOA, and county inspectors - not a city building department - enforce wind-load compliance. We’ve seen projects delayed two weeks because a window shipment arrived with an expired NOA certificate, or because a roofing underlayment didn’t match the approved assembly exactly. Contractors who don’t build regularly in unincorporated Miami-Dade underestimate this layer. We don’t.
Then there’s the water. Westchester’s wet season runs June through September, with afternoon thunderstorms that stop exterior work cold. Our schedules build in weather days, and we don’t pour slabs or set prefab units during tropical storm watches. The shallow limestone also means drainage and percolation tests for your septic or dry well can’t be fudged - we’ve had to redesign foundations after test holes showed impermeable rock at 14 inches.
Many Westchester lots were platted with minimal side setbacks, so squeezing in a detached unit while maintaining the required 5-10 foot fire separation often requires creative siting or a variance request. We handle that survey and filing as part of our pre-construction package.
If your project is part of a full property update, our Home Additions & ADU in Westchester page covers the broader scope. Our Home Additions & ADU crew coordinates every phase, and we serve homeowners throughout Westchester and nearby Flagami, West Miami, and Coral Terrace. Many clients pair their ADU with interior work - our Kitchen Design & Layout in Westchester and Backsplash, Tile & Flooring in Westchester teams can refresh the main house while the unit goes up out back.
Serving Westchester, FL - Our Local Coverage Area
We’re based in the Westchester area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
Frequently Asked - New Detached ADU in Westchester
Most new detached ADU projects in Westchester take 4-7 months from permit approval to certificate of occupancy, with prefab builds on the shorter end and stick-built or complex foundation jobs on the longer end. Permitting itself adds 4-8 weeks due to Miami-Dade’s HVHZ engineering review. Call (844) 665-0162 and we’ll give you a realistic timeline based on your lot and build type.
Yes - every new detached ADU in Westchester requires Miami-Dade County building permits, zoning approval, and multiple inspections because the area sits in the High Velocity Hurricane Zone. There is no city-level permitting option; unincorporated Miami-Dade handles everything through its central office. We manage the entire submission and inspection schedule as part of our project management.
Sometimes, but many Westchester homes built before 1980 have 100-amp panels that cannot safely handle an additional dwelling unit’s load. We assess your main panel capacity during the initial site survey and tell you upfront if a service upgrade is required. That typically adds $4,000-$8,000 and 2-3 weeks for FPL coordination.
Prefab units from Mighty Buildings, Boxabl, or Plant Prefab usually cost 10-20% less than comparable stick-built construction in Westchester, but the savings shrink if your site needs extensive crane access prep or foundation work. The real advantage is speed - 6-10 weeks less on-site construction. We’ll run both numbers for your lot so you can decide.
Our workmanship warranty covers structural and MEP installation for a defined period, and every product we install carries its own manufacturer warranty - roofing from GAF or CertainTeed, windows from Andersen or Pella, cabinets from KraftMaid or Wellborn. We provide all warranty documents at closing and register products on your behalf where required. For exact coverage terms on your specific material package, call (844) 665-0162.
Get a Written Estimate for Your New Detached ADU in Westchester
We’ll walk your lot, check zoning and utility access, and deliver a detailed written estimate with real pricing and a realistic schedule - no deposit required to start the conversation. Call (844) 665-0162 or reach out through our site. Raymond Estevez and our Westchester team will handle every phase, from permit to final inspection, as your single-source contractor.
Written by Raymond Estevez, Owner at Groundwork Construction Co., serving Westchester since our founding.