Small spaces are unforgiving. In a 3,000 sq ft house, a badly placed door is an annoyance. In a 400 sq ft cottage, it’s the whole problem.

A small ADU doesn’t need more square feet. It needs fewer wasted ones.

Below are three proven small-ADU layouts, described in plain words, plus how each one lines up with local size limits.

Start with the size limit, not the wish list

Your city sets the ceiling before you draw a single wall:

• City of Tampa, Extended Family Residence (EFR): family members only, up to 600 sq ft, approved through Special Use. This is the path for most Tampa neighborhoods.
• City of Tampa, full ADU: allowed only in designated areas, up to 950 sq ft.
• Unincorporated Hillsborough County: living space capped around 900 sq ft.
• St. Petersburg: up to 800 sq ft in NT and NS districts.

How a jurisdiction measures square footage (and whether a porch counts) can vary, so confirm with your city before you finalize the plan. The full details are in our Tampa, Hillsborough, and St. Pete ADU rules guide.

The 400 sq ft studio

Picture a rectangle about 20 by 20 feet. You enter into the living area, with a compact kitchen along one wall. The bed sits in an alcove on the far side, partly screened by a half wall or a run of tall storage cabinets. The bathroom and a closet share one wall, which keeps the plumbing in one place and the cost down.

What makes it work: one wall of tall cabinets, a pocket or barn-style door to the bathroom so no swing eats floor space, and windows on at least two sides so it doesn’t feel like a box.

Best for: a single, independent adult, a guest suite, or a home office and studio. It’s tight for a full-time resident who uses a wheelchair, because the 60-inch turning circle in the bath and kitchen takes real room at this size.

The 600 sq ft one-bedroom

Think about 24 by 25 feet. The front door opens to an open living and kitchen space with room for a small dining table. A short hall, or no hall at all, leads to a separate bedroom with a real closet. The bathroom sits between the bedroom and living area, with a door from each side or a door off a small landing.

What makes it work: a door that closes between sleeping and living, a bathroom big enough for a curbless shower and a 60-inch turning circle, and a stacked washer and dryer in a closet.

Best for: a parent living full time. This is the sweet spot for Tampa’s EFR path, because it fits the 600 sq ft cap without feeling like a compromise.

The 800 sq ft one-bedroom plus den

Picture about 25 by 32 feet, or an L-shape wrapped around a porch. You get everything in the 600 sq ft plan, plus a den of around 100 square feet off the living area. The den can be an office, a hobby room, a guest bed, or a room for an overnight caregiver. Often there’s room for a half bath near the den, too.

What makes it work: putting the den where it can borrow light from the living area, and giving it a door so it can become a real bedroom later.

Best for: a couple, a parent who may need a caregiver, or anyone who works from home. It matches St. Pete’s 800 sq ft max and fits under Tampa’s 950 sq ft limit for full ADUs.

The three plans side by side

	400 sq ft studio	600 sq ft one-bedroom	800 sq ft one-bed + den
Rough footprint	About 20′ x 20′	About 24′ x 25′	About 25′ x 32′ or L-shape
Bedroom	Alcove, open to living	Separate, with door	Separate, plus flexible den
Bathroom	One, compact	One, room for curbless shower	One full, often space for a half bath
Kitchen	Wall kitchen or kitchenette	Full small kitchen	Full small kitchen with more counter
Laundry	Often shared with main house	Stacked unit in a closet	Stacked unit or small laundry closet
Fits Tampa EFR (600 max)?	Yes	Yes	No
Fits St. Pete (800 max)?	Yes	Yes	Yes
Fits Tampa full ADU (950 max)?	Yes	Yes	Yes
Best for	One person, guest suite, office	A parent living full time	A couple, a caregiver, a home office

Design tips for small ADUs

Storage is a floor plan decision

In a small unit, closets don’t get added at the end. Plan full-height cabinets, a real coat closet by the door, and storage under window seats. Every item without a home ends up on the floor.

Treat the porch like a room

In Tampa Bay, a covered porch is usable most of the year. A deep porch, about 8 feet or more, holds two chairs and a small table, and it makes 600 sq ft feel like a lot more. Check whether your city counts it toward the size limit.

Design for aging from the start

Wide doors, a zero-step entry, a curbless shower, and grab-bar blocking don’t take extra square footage if you plan them in from the first sketch. Our universal design spec sheet lists every spec.

Keep the plumbing together

Backing the kitchen and bathroom onto the same wall shortens pipe runs and simplifies the build. On a tight budget, that matters.

For what these plans cost to build, see our ADU cost guide. Quality ADUs in Tampa Bay generally run $175,000–$350,000 all-in, and every lot is different.

Frequently asked questions

What is the best size for an ADU for a parent?

For most parents, a 600 square foot one-bedroom is the sweet spot. It gives a separate bedroom, an accessible bathroom, and a full small kitchen, and it fits the City of Tampa’s 600 square foot limit for an Extended Family Residence.

How big can an ADU be in Tampa?

A full ADU in Tampa’s designated areas can be up to 950 square feet. Everywhere else in the city, an Extended Family Residence for family members is limited to 600 square feet. St. Petersburg allows up to 800 square feet, and unincorporated Hillsborough County caps living space around 900.

Is 400 square feet enough for an ADU?

It can be for one independent adult, a guest suite, or a home office. A well-planned 400 square foot studio has a kitchen, bathroom, and sleeping alcove. It gets tight for full-time wheelchair use, since turning circles take real room at that size.

Does a porch count toward ADU square footage?

It depends on the jurisdiction and how the porch is built. Rules on measuring square footage vary, so confirm with your city or county before you finalize the plan.

Can a small ADU be wheelchair accessible?

Yes, with planning. A 600 square foot one-bedroom can fit 36-inch doors, a zero-step entry, a curbless shower, and 60-inch turning circles in the bath and kitchen. At 400 square feet it is possible but tight.