What ADU can you build in your Key West backyard?
Instant zoning report — see exactly what your zoning allows: permitted ADU types, size limits, setbacks, and lot requirements for your specific property.
The quick answer
Key West ADU rules at a glance
- Max ADU size
Up to 750 sq ft, and smaller than your main home - Owner-occupancy
Not required — you can rent both units - Short-term rentals
Allowed with local licensing — verify with the city - Where it's allowed
Eligibility varies by zoning district — check your address to confirm yours
ADU Rules in Key West, Florida
We've researched Key West's ADU rules so you don't have to. Here's what matters most — the facts that determine whether your project is possible, and how much it will cost.
- Key West is Florida's most constrained ADU market — but also its highest-income STR opportunity
- Monroe County ACSC designation imposes stricter growth management than the rest of Florida — confirm local ADU rules with city planning
- STR Vacation Rental licenses are capped — verify license availability before buying for STR income; some licenses trade separately for $50k–$200k+
- Construction costs are the highest in Florida: $200,000–$450,000 for a 500–750 sq ft unit due to island logistics and hurricane standards
- Rental income potential is exceptional: $2,200–$5,500/month LTR; $250–$700/night STR for a well-positioned unit near Old Town
- Old Town Historic District (most of Key West) requires HARC Certificate of Appropriateness for any new structure
Last verified: 2026-04-02
Backyard ADUs like this are being built across Key West — here's how to do it right.
What Key West's zoning may allow for an ADU
Key West ADU rules. Monroe County is Florida's Area of Critical State Concern — a state designation that limits growth and creates unique regulatory constraints. There is no statewide ADU preemption, so local rules govern. Extremely high land values and STR demand.
Zoning allowance
ADUs may be permitted on eligible single-family parcels, but Monroe County is designated as a Florida Area of Critical State Concern (ACSC) — this imposes growth management restrictions that differ from the rest of Florida. Verify ADU eligibility for your specific parcel and zone with the City of Ke
Size limits
City of Key West typically limits accessory dwelling units to 550–750 sq ft in historic districts. Verify current limits with Key West Planning; Florida has no statewide ADU size floor, so local limits govern.
Lot + setbacks
Key West lots are among the smallest in Florida — typical lots 40–60ft wide, 80–100ft deep. Setbacks: 5ft sides, 10ft rear in most residential zones. Many properties are in the Old Town National Register Historic District requiring additional design review.
Height + coverage
Maximum height 25–35ft. Historic Key West requires architectural compatibility — no flat-roofed modern structures in Conch cottage neighborhoods.
Parking
Parking is critically constrained in Key West — many properties have no off-street parking. ADU parking requirements may be waived in Old Town, but confirm with planning.
Rental / occupancy
Key West has a complex STR licensing system with caps on transient rental units by zone. Residential areas require Vacation Rental (VR) license — licenses are limited by city ordinance. Confirm availability before purchasing for STR income.
Permit process
City of Key West Building Department. Historic Old Town properties require HARC (Historic Architecture Review Commission) Certificate of Appropriateness. Monroe County Building for unincorporated Keys parcels.
Building limits and rental rules, side by side
These figures are based on the best available data for Key West and should be confirmed with the city's planning department before you commit to a design or a rental strategy — local ordinances change and vary by parcel.
Building Limits
- Max Size:750 sq ft
- Setbacks:5ft side / 10ft rear
- Detached Allowed?Yes
- Max Height:35 ft
- Build Cost Est.:$200,000–$450,000
Rental Rules
No Owner-Occupancy Requirement: Both units can be rented to tenants simultaneously.
Short-Term Rentals: Not prohibited at the city level. State DBPR license required.
Estimated Monthly Income
$2,200–$5,500/month
Based on 2025–2026 Key West market data
ADUs are NOT Permitted In:
- Key West Naval Air Station and adjacent federal land — no civilian ADU development
- Salt Ponds and environmentally sensitive habitat areas
- FEMA Zone VE (coastal high hazard) properties — extremely restrictive for new construction
- Commercial zones along US-1 corridor — not eligible for residential ADUs
Here's exactly what you'll pay — and how to avoid the fees that catch most homeowners off guard
Most homeowners spend 3–4 weeks piecing this together on their own. We've mapped the full Key West permit process step by step — so you know exactly what's coming, and what to prepare.
Step 1: Pre-Application Zoning Check
- Where:
- City of Key West Planning Department, 3140 Flagler Avenue, Key West, FL 33040
- Contact:
- (305) 809-3890
- What to bring:
- Property address and parcel ID, preliminary site sketch, lot survey
- Timeline:
- No appointment fee — call ahead to confirm office hours
- Cost:
- No fee for initial inquiry
Step 2: Submit Permit Application
- Where:
- https://www.keywestcity.com/274/Building-Department
- Contact:
- (305) 809-3890
- What to bring:
- Architectural plans, site plan, boundary survey, energy compliance docs
- Timeline:
- Standard zones: 8–14 weeks
- Cost:
- $11,100–$26,800 in permits and fees (excluding construction)
Step 3: Plan Review & Revisions
- Where:
- City of Key West Planning Department
- Contact:
- (305) 809-3890
- What to bring:
- Responses to any correction letters; revised drawings if required
- Timeline:
- Standard zones: 8–14 weeks. Old Town Historic District: add 6–12 weeks for HARC review. Construction complexity adds additional time.
- Cost:
- No additional fee for standard resubmittal
Step 4: Construction & Inspections
- Where:
- Key West Building Department
- Contact:
- (305) 809-3890
- What to bring:
- Approved permit set must be on-site at all times during construction
- Timeline:
- Schedule inspections as each phase of work is completed
- Cost:
- Included in permit fee
Full Key West Permit Cost Breakdown
These are the fees you'll encounter, in the order you'll typically encounter them.
| Cost Item | Amount / Notes |
|---|---|
| Total Estimated Cost | $11,100–$26,800 in permits and fees (excluding construction) |
| Building Permit Fee | $3,000–$7,000 |
| harc review fee | $300–$800 (Old Town only) |
| Impact Fees | $5,000–$12,000 |
| Plan Review Fee | $800–$2,000 |
| Utility Connection Fees | $2,000–$5,000 |
Key ADU Restrictions in Key West
These are the rules that most often catch homeowners by surprise. Read them carefully before engaging an architect.
- Monroe County is designated a Florida Area of Critical State Concern (ACSC) — growth management restrictions limit new construction; confirm local ADU rules with city planning
- STR (Vacation Rental) licenses are capped by city ordinance in Key West — the city limits the total number of transient rental units; confirm a license is available before purchasing a property for STR income
- Old Town National Register Historic District covers most of downtown Key West — HARC review required for all exterior changes, including new ADU structures
Common Mistakes to Avoid in Key West
- Key West Vacation Rental licenses are a limited, transferable asset — some sell for $50,000–$200,000+ separately from the property itself; always clarify whether a VR license transfers with a sale
- Hurricane hardening requirements are the strictest in Florida — all new construction must meet Miami-Dade NOA (Notice of Acceptance) standards for wind; this adds significant cost to ADU construction
- Many Key West lots have underground cisterns, cistern lids, or buried infrastructure from the pre-WWII era that can surprise during construction — require a geotechnical report
Eligible Zones in Key West
ADUs may be permitted in these zoning districts, but eligibility depends on your specific parcel and the current local ordinance. Confirm your parcel and rules with Key West Planning before relying on this — most homeowners can check their zone at the county GIS portal in a few minutes.
Key West neighborhoods where ADU permits are being filed — is your street next?
Key West ADU — Frequently Asked Questions
These are the questions we hear most often from Key West homeowners. If yours isn't here, our ADU Zoning Experts can answer it directly.
How do Florida ADU rules apply in Key West?
This is the most important question for Key West ADU planning and the answer is uncertain. Monroe County is designated a Florida Area of Critical State Concern (ACSC) under Florida law, which means the county operates under a state-approved Comprehensive Plan with stricter growth management rules than the rest of Florida. Florida has no statewide ADU preemption, so Key West's local caps govern. Do not assume ADU rights in Key West without confirming with the City of Key West Planning Department or a Florida land-use attorney.
How do STR Vacation Rental licenses work in Key West?
Key West limits the total number of Vacation Rental (VR) licenses it issues in residential areas. These licenses are transferable, meaning they can be bought and sold — in some cases for $50,000–$200,000 independently of the property. A VR license is required to legally operate a short-term rental (under 28 days) in Key West residential zones. Before purchasing any Key West property for STR income, confirm: (1) the property currently has a VR license or (2) a new license is available in the city's allocation. Operating without a license carries significant fines.
Why are construction costs so high in Key West?
Key West construction costs are 2–3x mainland Florida for several reasons: (1) Island logistics — all materials arrive by truck over the Overseas Highway or by barge, adding 15–25% to materials costs; (2) Hurricane hardening — all new construction must meet Miami-Dade NOA wind standards, among the strictest in the country; (3) Historic materials — Old Town ADUs must use compatible materials (Dade County pine, painted wood siding, metal roofs) that cost more than modern alternatives; (4) Local labor scarcity — skilled tradespeople are limited on the island.
What is the HARC review and does my ADU need it?
The Historic Architecture Review Commission (HARC) reviews all exterior changes to properties in the Key West Historic District, which covers most of the original city. If your property is in the historic district (check with Key West Planning), you must receive a Certificate of Appropriateness from HARC before pulling a building permit for any new structure including an ADU. HARC reviews massing, materials, roof form, window placement, and compatibility with the Conch architectural vernacular. Work with an architect who has HARC experience.
Key West Planning Department
We recommend calling City of Key West Planning Department at (305) 809-3890 before submitting anything — a 10-minute conversation can save weeks of back-and-forth on your permit application.
City of Key West Planning Department
Nearby Cities — ADU Rules
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This page provides general information based on current ordinances and publicly available data as of April 2026. Zoning rules change — verify all requirements with your local planning department before making financial commitments.