If you've confirmed your property qualifies for an ADU — the next question is whether the numbers work. That depends on which ADU type fits your lot, what St. Pete's size limits allow, and whether rental income makes the investment pencil out.

This guide covers ADU cost by type, the St. Pete-specific rules that shape your cost ceiling, the Florida factors that push costs above national estimates, and the rental income math. For eligibility specifics, read our guide on ADU eligibility requirements in St. Petersburg.

1. The Direct Cost Answer

ADU construction in St. Petersburg costs $90,000–$200,000+ depending on type. Detached ADUs built from the ground up sit at the high end. Garage conversions using existing structure sit at the low end.

2. ADU Cost by Type

The ADU type determines the floor of your cost more than any other single variable.

3. St. Pete Ordinance Rules That Shape Your Cost Ceiling

St. Petersburg's ADU ordinance directly limits how much you can build — and therefore how high your cost can realistically go.

• 800 square foot maximum — no ADU in St. Pete can exceed 800 sq ft of floor area; this caps your maximum construction cost regardless of budget
• 67% rule — the ADU cannot exceed 67% of the principal dwelling's floor area; on smaller homes this may limit you below the 800 sq ft ceiling
• Owner-occupancy requirement — either the main home or the ADU must be owner-occupied; investors cannot buy a property solely to rent both units
• Setback requirements — detached ADUs must meet setback rules from property lines; on smaller lots this constrains placement and sometimes size
• Rental is permitted — the City explicitly allows long-term rental of the ADU; confirm current short-term rental rules separately before designing for that use

For full eligibility and zoning details, read our guide on ADU eligibility requirements in St. Petersburg.

4. Florida-Specific Cost Factors

Four Florida factors push ADU construction costs above national estimates. None appear in generic online ADU cost calculators.

• Impact-rated windows and doors — required by Florida Building Code on all new residential construction; cost 30–50% more than standard products; on a small ADU with multiple openings, this is a meaningful line item
• Hurricane compliance engineering — all Pinellas County construction must be engineered for 130–160 mph wind speeds; ADUs require stamped structural drawings covering wind load, roof-to-wall connections, and hurricane strapping
• FEMA flood zone implications — if your lot is in an AE or VE flood zone, any new structure (including a detached ADU) must meet base flood elevation requirements; this can add $10,000–$25,000+ for elevated foundation construction
• Coastal material specifications — roofing, exterior finishes, and hardware must be specified for salt air and UV exposure; budget-grade materials degrade faster in coastal Florida conditions

5. The Rental Income Math

At current Tampa Bay market rents, a well-built ADU can generate meaningful monthly income — but the payback timeline depends on construction cost and achievable rent for your location and unit type.

6. ADU vs. Room Addition — Which Makes More Sense?

An ADU and a room addition serve different purposes. The right choice depends on whether you want to add private, income-generating space or expand your own living area.

• Choose an ADU if — you want to generate rental income, house a family member with privacy, or create a long-term asset that functions independently from the main house
• Choose a room addition if — you want to expand your own living space, add a master suite, or increase the functionality of your existing floor plan without a separate unit
• Both require permits — the Pinellas County plan review process and Florida Building Code requirements apply equally to both