What Are the Different Types of ADUs in St. Petersburg, FL?

At a Glance

ADU types permitted
Detached, attached addition, internal / garage conversion
Detached ADU cost
$130,000 – $200,000+
Attached ADU cost
$100,000 – $160,000+
Garage / internal conversion cost
$60,000 – $120,000
Maximum size (all types)
800 sq ft and no more than 67% of principal dwelling floor area
Owner-occupancy required
Yes — main home or ADU must be owner-occupied
Permit required
Yes — all three types require Pinellas County plan review

Most homeowners researching ADUs in St. Pete focus on eligibility first — can I build one? Once that's confirmed, the next question is which type fits the property. The answer depends on lot size, existing structures, setback clearance, and how you plan to use the unit.

This guide covers all three permitted ADU types in St. Petersburg — what each one is, what it costs, what it requires, and which property situations each fits best. For eligibility specifics, read our guide on ADU eligibility requirements in St. Petersburg.

Check out our ADU Construction in St. Petersburg

Types of ADUs in St. Petersburg FL

Quick Links

1. The Three ADU Types St. Petersburg Allows

St. Petersburg's ADU ordinance permits three distinct types — each defined by its relationship to the main home and how it is constructed.

ADU Type Structure Typical Cost Best For
Detached ADU Separate building on the same lot $130,000 – $200,000+ Maximum privacy, highest rental value
Attached ADU addition Connected to main home, separate entrance $100,000 – $160,000+ Shared wall reduces cost; family housing
Internal / garage conversion Existing space converted to habitable unit $60,000 – $120,000 Lowest cost; existing structure reused

2. Detached ADU

A detached ADU is a completely independent structure built on the same lot as the main home. It has its own foundation, framing, roof, and all utilities. No walls are shared with the primary dwelling. It functions as a fully self-contained living unit — as private as a separate home.

What a Detached ADU Is Best For

  • Rental income — maximum privacy for a tenant; a detached unit commands higher rent than an attached or conversion unit
  • In-law or family housing — complete independence from the main home; separate entrance, outdoor space, and daily routines that don't intersect
  • Long-term asset value — appraisers treat a detached ADU as a separate income-generating unit, which can increase the property's assessed value more than other ADU types

What a Detached ADU Requires

  • Adequate lot space — the detached structure must meet all setback requirements from property lines; on smaller St. Pete lots, setback constraints can make a detached ADU infeasible
  • New foundation — poured slab or elevated foundation depending on flood zone status; if the lot is in an AE or VE FEMA zone, the foundation must meet base flood elevation requirements
  • Full utility connections — electrical, plumbing, and HVAC must be extended to the new structure; these are real cost line items that don't exist for conversion ADUs
  • Florida Building Code compliance — impact-rated windows and doors, 130–160 mph wind load engineering, and hurricane strapping at every structural connection
  • Building permit — submitted electronically through the City of St. Petersburg's portal; Pinellas County plan review applies

Detached ADU Cost in St. Petersburg

$130,000–$200,000+ is the typical range for a detached ADU in St. Petersburg. The wide range reflects lot conditions, size (up to 800 sq ft maximum), finish level, and whether flood zone compliance requires an elevated foundation. At the high end, a fully finished 800 sq ft detached unit with premium finishes on a flood zone lot will exceed $200,000.

3. Attached ADU Addition

An attached ADU shares at least one wall with the main home but operates as a fully independent living unit. It has its own entrance — separate from the main home's entry — its own kitchen, bathroom, and living space. It is structurally connected to the primary dwelling but legally and functionally separate.

What an Attached ADU Is Best For

  • Family housing with proximity — closer daily connection than a detached unit; suited for aging parents or adult children where some shared oversight is desired
  • Smaller lots — doesn't require as much open yard space as a detached structure; can be built where lot dimensions or setbacks prevent a detached option
  • Lower cost than detached — sharing a wall reduces framing and exterior costs; utility connections can be shorter or partially shared

What an Attached ADU Requires

  • New foundation for the addition footprint — even though one wall is shared, the new portion requires its own slab or footing; flood zone rules apply to any new ground floor footprint
  • Separate entrance — required by ordinance; the ADU must have its own exterior door that does not pass through the main home's living space
  • Sound separation — the shared wall must meet building code requirements for sound isolation between residential units
  • Full mechanical independence — separate HVAC, electrical metering, and plumbing is standard for a rentable attached ADU; shared systems complicate tenancy and utility billing
  • Building permit — same Pinellas County plan review process as all ADU types

Attached ADU Cost in St. Petersburg

$100,000–$160,000+ is the typical range. The cost reduction vs. detached comes primarily from the shared wall and shorter utility runs. Foundation cost is similar since a new slab is still required for the addition footprint. Flood zone lots can push the range higher if the ground floor addition requires elevation.

4. Internal Conversion and Garage Conversion

A conversion ADU transforms an existing portion of the property into a self-contained living unit without adding new square footage to the lot's footprint. The two most common forms in St. Petersburg are garage conversions and internal conversions of unused rooms or basement-adjacent spaces.

Garage Conversion ADU

The most common conversion type. The garage structure — slab, walls, roof — is reused. Garage doors are replaced with impact-rated windows and a proper exterior entry. The space is insulated, finished, and fitted with a kitchen, bathroom, and HVAC.

  • Lowest upfront cost — existing foundation and structure eliminate the largest cost items in new ADU construction
  • Trade-off: you lose your garage — parking and storage must be accounted for; this affects some homeowners' decisions more than others
  • Florida Building Code upgrade required — the converted space must meet habitable room standards: insulation R-values, egress windows, impact-rated openings, and HVAC that can maintain 75°F or cooler in Florida's climate
  • Permit still required — change of use from garage to habitable space triggers a full permit application and Pinellas County plan review

Internal Conversion ADU

A portion of the existing home — typically a wing, a finished basement-level space, or an oversized room configuration — is reconfigured into a self-contained unit with its own entrance, kitchen, and bathroom. No new exterior construction required.

  • Least disruptive to the property — no new footprint, no exterior construction affecting yard or landscaping
  • Requires adequate existing square footage — the existing home must have space that can be physically separated into a functional unit while leaving the main home with sufficient living area
  • Egress requirements — the converted unit must have compliant egress windows and a separate exterior entrance; retrofitting egress into an existing structure can add cost

Conversion ADU Cost in St. Petersburg

$60,000–$120,000 is the typical range for garage or internal conversions. Primary cost items are insulation and weatherproofing upgrades to meet FBC habitable room standards, impact window installation, kitchen and bathroom addition, HVAC system, and the egress and entrance work. The slab and structural shell are already there — that's where the savings come from.

5. St. Pete Ordinance Rules That Apply to All Three Types

Regardless of which ADU type you build, the same St. Petersburg ordinance rules apply. These are not optional — and they must be confirmed for your specific property before any design or budgeting begins.

  • 800 square foot maximum — no ADU in St. Petersburg may exceed 800 sq ft of floor area; this applies to all three types equally
  • 67% rule — the ADU floor area cannot exceed 67% of the principal dwelling's floor area; on smaller homes, this cap may fall below the 800 sq ft ceiling
  • Owner-occupancy requirement — either the main home or the ADU must be owner-occupied; the property cannot be investor-owned with both units rented
  • Setbacks — all ADU types must meet the applicable setback distances from property lines; detached ADUs are most affected; internal conversions are least affected
  • Separate entrance required — all three types must have an exterior entrance that does not pass through the main home's primary living space
  • Permit required for all types — there is no permit exemption for any ADU type in St. Petersburg; all require electronic submission and Pinellas County plan review
⚠ Confirm your property's eligibility before choosing a type. Lot coverage limits, setback constraints, and flood zone status can eliminate some ADU types on specific parcels — regardless of what the ordinance generally permits. A site assessment is the first step, not the last.

6. Which ADU Type Is Right for Your Property?

The right ADU type is determined by your lot, your budget, and how you plan to use the unit. Use this framework before committing to a design.

Choose a Detached ADU If:

  • Your lot has adequate space and setback clearance for a separate structure
  • Maximum privacy for occupants is a priority — rental or independent family housing
  • You want the highest rental value and long-term asset impact
  • Your budget can accommodate $130,000–$200,000+ in construction cost

Choose an Attached ADU If:

  • Your lot is smaller or setback constraints prevent a detached structure
  • You want proximity to occupants — aging parents, adult children, or a caregiver situation
  • You want to reduce cost vs. detached while still adding a fully independent unit
  • You have space on the side or rear of the home that can accommodate an addition

Choose a Conversion ADU If:

  • You have an existing garage or interior space that can be converted
  • Your primary goal is lowest upfront cost while still creating a rentable or usable unit
  • Your lot doesn't have room for a new structure or addition
  • You're comfortable trading garage storage and parking for the ADU income or use

Frequently Asked Questions

What types of ADUs are allowed in St. Petersburg, FL?
St. Petersburg allows three types of ADUs: detached ADUs (a separate structure on the same lot), attached ADUs (an addition connected to the main home with a separate entrance), and internal conversions including garage conversions (existing space converted to a self-contained unit). All three types are subject to the 800 sq ft maximum, the 67% rule, and the owner-occupancy requirement.
What is the difference between a detached and attached ADU in Florida?
A detached ADU is a completely separate structure — its own foundation, walls, roof, and utilities. An attached ADU shares at least one wall with the main home but has its own entrance, kitchen, and living space. Detached ADUs offer more privacy and typically higher rental value. Attached ADUs cost less because they share structural elements with the main home.
Can I convert my garage into an ADU in St. Petersburg?
Yes — garage conversions are a permitted ADU type in St. Petersburg. The garage must be converted into a self-contained unit with a kitchen, bathroom, and separate entrance. It must meet Florida Building Code requirements for habitable space — insulation, egress windows, impact-rated openings, and adequate HVAC. A building permit is required. The property must meet the owner-occupancy requirement.
Which type of ADU is cheapest to build in St. Petersburg?
A garage or internal conversion is the least expensive ADU type, typically $60,000–$120,000. It reuses existing structure and foundation, eliminating the largest cost items in new ADU construction. A detached ADU is the most expensive at $130,000–$200,000+ but delivers the most privacy, independence, and rental value.