Junior ADU & Interior Conversion Cost - Design, Permits and Build Under One Contract

How Much Does Junior ADU & Interior Conversion Cost in Long Beach?

A junior ADU (JADU) conversion in Long Beach typically costs between $45,000 and $90,000 when built within existing square footage, while a full attached ADU conversion runs $80,000 to $150,000. Interior conversions that repurpose a garage, bonus room, or underused wing of the house sit on the lower end because the shell, roof, and foundation already exist. You’re paying for permits, plumbing, electrical, HVAC, drywall, fixtures, and finish work rather than new construction from the ground up.

That’s the honest range. Contractors who quote $20,000 for a JADU are cutting corners somewhere, and a $250,000 interior conversion means you’re doing a whole lot more than a conversion. Let’s break down exactly where the money goes so you can plan with real numbers, not vague promises.

This guide pulls from current Long Beach permit fees, local labor rates, and the actual bids our team at Benson ADU Long Beach has written for homeowners in neighborhoods like Bixby Knolls, Belmont Heights, and North Long Beach. We believe you should know the ballpark before you ever tap “call.”

Key Takeaways

• JADU (up to 500 sq ft, inside existing walls): $45,000 to $90,000, permits included
• Attached ADU conversion (garage, bonus room, basement): $80,000 to $150,000
• Larger interior conversions (800+ sq ft): $130,000 to $200,000+
• Long Beach permit fees for ADUs: roughly $4,000 to $9,000 depending on scope and school district fees
• Timeline: 3 to 6 months for most conversions, with prefab interior kits sometimes cutting site time by half
• Written price before work starts: this is the single biggest factor separating a clean project from a financial headache

Junior ADU & Interior Conversion Cost Breakdown (2026)

Here’s what a realistic budget looks like for a JADU or interior conversion in Long Beach. These numbers reflect permits, materials, labor, and typical finish levels (not builder-grade, not luxury). Adjust upward if you’re adding a kitchen with high-end appliances or a fully tiled bathroom with custom glass.

Cost Component	JADU (up to 500 sq ft)	Attached ADU Conversion (500 – 750 sq ft)	Larger Interior Conversion (800+ sq ft)
Design & engineering	$3,000 – $6,000	$5,000 – $9,000	$7,000 – $12,000
Long Beach permits & fees	$4,000 – $6,500	$5,000 – $8,000	$6,000 – $9,000
Demolition & site prep	$2,000 – $4,000	$3,000 – $6,000	$5,000 – $9,000
Plumbing (rough + finish)	$6,000 – $12,000	$9,000 – $16,000	$12,000 – $20,000
Electrical (panel upgrade, wiring, fixtures)	$5,000 – $10,000	$7,000 – $14,000	$9,000 – $18,000
HVAC (mini-split or tied to existing)	$3,500 – $6,000	$4,500 – $8,000	$6,000 – $12,000
Insulation & drywall	$4,000 – $7,000	$6,000 – $10,000	$8,000 – $14,000
Kitchenette or full kitchen	$8,000 – $18,000	$12,000 – $25,000	$18,000 – $40,000
Bathroom (full or 3/4)	$12,000 – $22,000	$15,000 – $28,000	$18,000 – $35,000
Flooring, paint, trim, doors	$4,000 – $8,000	$6,000 – $12,000	$8,000 – $16,000
Contractor overhead & margin	15 – 20% of total	15 – 20% of total	15 – 20% of total
Typical total range	$45,000 – $90,000	$80,000 – $150,000	$130,000 – $200,000+

Now, what moves the needle up or down from those midpoints? The biggest driver in Long Beach is whether you’re converting already-conditioned space (a spare bedroom, a den, an enclosed sunroom) or converting a garage that was never insulated, never drywalled, and never wired for a kitchen or bath. Garages almost always need a slab inspection, vapor barrier work, and full electrical upgrades, and that adds $15,000 to $25,000 fast. A bonus room above the garage or a first-floor bedroom suite converts for less because the bones are largely there.

Prefab interior kits, which we offer through brands like Abodu, Cover, and Boxabl, can compress the finish phase and lock in material pricing earlier. Instead of waiting on custom cabinetry or backordered tile, the panels and fixtures arrive pre-cut and pre-finished. Homeowners who choose this path in Long Beach generally save 10 to 15 percent on finish labor and shave three to six weeks off the schedule.

What Affects Junior ADU & Interior Conversion Pricing in Long Beach

• Existing condition of the space. A finished bonus room with drywall, insulation, and a subpanel converts for far less than a 1960s garage with a cracked slab and no insulation. In older Long Beach neighborhoods like Rose Park or Willmore City, we regularly see galvanized plumbing and cloth-wrapped wiring that has to be replaced before framing even starts.
• Foundation and slab work. Garages built before the mid-1970s often sit on slabs that were never meant to support living space. If the slab needs reinforcement, epoxy sealing, or a moisture barrier, add $4,000 to $10,000. Coastal areas like Belmont Shore and the Peninsula are more prone to moisture intrusion, and that affects what the building inspector will require.
• Electrical panel capacity. Long Beach homes built in the 1950s and 1960s often have 100-amp panels. A JADU with a kitchenette, bathroom, and mini-split typically needs a 200-amp service upgrade, which runs $3,500 to $6,500 depending on how far the panel sits from the meter and whether Southern California Edison requires a new drop.
• Kitchen vs. kitchenette. A JADU is legally permitted to have a “kitchenette” with a small refrigerator, a microwave, and a one- or two-burner cooktop. Full ADUs require a full kitchen with a range, vent hood, and more counter space. That distinction alone changes the cost by $8,000 to $15,000.
• Setbacks and egress requirements. For attached conversions, setbacks rarely apply because you’re inside an existing structure. But egress windows, fire-rated doors between the main house and the new unit, and dedicated exterior access all carry hard costs. A fire-rated door assembly runs $800 to $1,800 installed, and a proper egress window in a converted garage is $1,200 to $3,000.
• Finish level. Flooring, cabinets, tile, and fixtures can swing the whole project by $15,000 or more. LVP flooring runs $6 to $10 per square foot installed in Long Beach. Engineered hardwood runs $12 to $18. Quartz countertops run $70 to $110 per square foot installed. None of this is hidden, but it’s where budget creep lives if nobody writes it down before the first wall comes out.

One more factor that doesn’t show up in a spreadsheet: whether your contractor gives you a written price before work starts. Oral estimates in Long Beach routinely become change orders three weeks in. That is exactly why we put every price in writing on the first conversation, governed by The Relay Visibility Guarantee. No surprises at the door, no scope creep billed after the fact. If the work isn’t right within 365 days, we come back and make it right, in writing, on every project.

Long Beach Permit Costs and Timing

Long Beach runs its own building department, and ADU permit fees here are not the same as in unincorporated Los Angeles County or neighboring cities like Lakewood or Signal Hill. For a typical JADU or interior conversion, plan on $4,000 to $9,000 in total permit and impact fees. That includes plan check, building permits, electrical, plumbing, mechanical, and school district impact fees where they apply.

Plan check for an ADU conversion in Long Beach generally takes 3 to 6 weeks from submittal to approved plans, assuming your drawings are clean and complete. Over-the-counter permits for simple JADUs with no structural changes can move faster, but most interior conversions require full plan review because they touch electrical, plumbing, and egress.

We don’t promise a specific permit timeline for any single Long Beach neighborhood, because the city’s review queue changes month to month. What we do promise is that your plans are submitted complete the first time. Resubmittals are the number one schedule-killer in Long Beach ADU projects, and they cost homeowners both time and revision fees.

How to Save on Junior ADU & Interior Conversion

1. Convert a space that’s already conditioned. The single biggest savings lever. A spare primary suite, a den, or a finished bonus room skips foundation work, insulation, and major envelope repairs. You might save $20,000 to $40,000 versus a garage conversion.
2. Keep it a JADU, not a full ADU. The smaller the square footage and the simpler the kitchen, the lower the city fees and the lower the construction cost. A 450-square-foot JADU with a kitchenette costs materially less than a 750-square-foot attached ADU with a full kitchen and separate laundry.
3. Choose prefab interior components. We build with Abodu, Cover, Dvele, Mighty Buildings, Boxabl, Plant Prefab, and Tuff Shed. Prefab panels and pre-finished interior kits arrive measured, cut, and ready for assembly, which reduces on-site labor, minimizes waste, and keeps material costs fixed. You can research every one of those brands yourself before making a decision.
4. Lock your finishes before demo day. The biggest budget overruns happen when homeowners change tile, flooring, or cabinet choices mid-project. Pick everything first, get it on a written allowance sheet, and don’t shop “just to see what else is out there” once the walls are open.
5. Use a contractor who gives you a written price. We lead with this because it’s the difference between a $75,000 project and a $95,000 project with the same square footage. Written price before work starts, real-time tracking during the build, and a digital invoice payable by link at the end. If a contractor won’t put the number in writing, walk.

If you want a precise number for your specific house and your specific conversion, call us at (562) 362-3885. Estimates are free, and you’ll get a written price, not a handshake.

Prefab vs. Site-Built for Interior Conversions

This is a question we get on nearly every Long Beach consultation, and it deserves a straight answer. Site-built means your conversion is framed, insulated, drywalled, and finished entirely on your property by our crews. Prefab means the structural components or interior modules are manufactured off-site in a controlled facility, then delivered and assembled at your home.

Prehab, when done well, is faster and often cleaner. Panels from Abodu and Cover arrive pre-cut with windows and doors already set. Interior kits through Boxabl and Plant Prefab bring entire bathroom and kitchen modules that get craned or rolled into place and connected. The finish quality is consistent because it was built indoors, not in Long Beach’s coastal humidity. James Hardie cladding and trim, which we use on many attached conversions, adds durability without the maintenance calendar of wood.

Site-built gives you more flexibility for irregular footprints and complicated existing conditions, which are common in older Long Beach homes with non-standard garage dimensions or structural quirks. Site-built also allows mid-course adjustments if you discover something unexpected behind an old wall. The tradeoff is time and some material price volatility.

Our clients frequently choose a hybrid: prefab shell or interior modules where they make sense, site-built custom work where they don’t. There’s no single right answer for every Long Beach house, which is why we bring both options to the table instead of a take-it-or-leave-it product line.

How Much Does a Garage Conversion Cost in Long Beach?

A garage-to-ADU conversion in Long Beach typically runs $85,000 to $150,000 for a 400 to 600 square foot unit. The range is wide because older garages in neighborhoods like California Heights and Los Altos were built as car storage, not living space. The slab is often thin, the walls uninsulated, the roof structure unbraced for ceiling loads, and the electrical feed is a single circuit. Converting that into a legal dwelling means upgrading all of it.

Expect the permit plan check to flag at least three of these items: slab moisture barrier, wall and ceiling insulation, panel capacity, egress, and fire separation from the main house. A fire-rated wall and door assembly between the house and the new unit is required by California building code and adds $2,000 to $5,000 even before finishes. If your garage has a laundry hookup or a bathroom rough-in, you’re ahead of the game. If it’s a bare box with a roll-up door, budget toward the top of the range.

If you’re weighing a garage conversion against a Junior ADU & Interior Conversion in Long Beach, the JADU route often wins on price per square foot. Call us at (562) 362-3885 and we’ll walk through both options for your specific property.

How Much Does a Bonus Room or Bedroom Suite Conversion Cost?

Converting an existing bonus room, den, or primary bedroom into a JADU or attached ADU runs $45,000 to $85,000 in Long Beach. This is the sweet spot for homeowners who want rental income or a family suite without structural gymnastics. The room already has walls, insulation, heating, and at least one window. You’re adding a kitchenette, a bathroom, a separate entrance, and the required fire separation.

A dedicated exterior entrance is a non-negotiable requirement for any legal ADU in California, and that single item costs $2,500 to $6,000 depending on whether you’re cutting a new opening or converting an existing window. A bathroom addition inside a bonus room typically runs $12,000 to $22,000 including plumbing, waterproofing, tile, and fixtures. The kitchenette adds $8,000 to $18,000. When those numbers are stacked, the $45,000 floor becomes clear quickly.

Some of the fastest, cleanest bonus room conversions we’ve completed in Long Beach used prefab bathroom modules from Dvele and Mighty Buildings. The module arrives pre-plumbed and pre-finished, the crew connects it, and the whole bathroom phase shrinks from three weeks to about five days. That’s the kind of efficiency that shows up in the final invoice.

Does a JADU Add Value to a Long Beach Home?

Yes, and the math is concrete. A completed JADU or attached ADU in Long Beach typically adds $80,000 to $150,000 in appraised value, based on recent comparable sales in neighborhoods like El Dorado Park, Bixby Knolls, and North Long Beach where ADUs have sold in the last two years. That assumes the unit is legal, permitted, and finished to a standard that appraisers recognize.

Unpermitted conversions, which are common in older Long Beach homes, add almost nothing to appraised value and create issues at sale. Buyers’ lenders won’t count unpermitted square footage, and sellers often have to either legalize the unit or disclose it as non-conforming. If you’re thinking about doing the work without permits to save money, don’t. The savings disappear at resale and the liability hangs around the whole time.

On the rental income side, a 500 square foot JADU in Long Beach rents for roughly $1,600 to $2,200 per month depending on location, parking, and finishes. That’s $19,000 to $26,000 per year in gross rent against a $50,000 to $80,000 construction cost. The return is real, which is why so many Long Beach homeowners are converting spare space right now.

What’s the Difference Between a JADU and an ADU in Long Beach?

A JADU is a junior accessory dwelling unit of no more than 500 square feet, created entirely within the existing walls of a single-family home or attached garage. It must have a separate entrance, a kitchenette (not a full kitchen), and may share a bathroom with the main house or have its own. A full ADU is a separate dwelling unit with its own kitchen, bathroom, and entrance, and it can be attached, detached, or converted from existing space.

The cost difference matters. Because a JADU can share a bathroom and doesn’t require a full kitchen, the plumbing and electrical scope is smaller. In Long Beach, a JADU conversion starts around $45,000 while a full attached ADU conversion starts closer to $80,000. The city treats both as legal dwellings with separate addresses, and both count toward your property’s ADU allowance under state law. Most Long Beach single-family lots are allowed one JADU and one detached ADU, or one full ADU created from conversion, depending on your specific lot and existing structures.

Can You Convert an Attached Garage into a JADU in Long Beach?

Yes. California state law and Long Beach municipal code both allow attached garage conversions for JADU or ADU use, and the city waived its old requirement to replace converted garage parking spaces. That means you can convert the entire attached garage without adding a new carport or covered parking. Long Beach is one of the more permissive cities in Los Angeles County on this point, and it’s a major reason attached garage conversions are so popular here.

The cost to convert an attached garage into a JADU in Long Beach ranges from $65,000 to $120,000. The lower figure assumes a single-car garage with good slab condition, simple geometry, and a nearby electrical panel with spare capacity. The higher figure assumes a two-car garage, slab repair, new panel, upgraded gas line, and full kitchen and bath.

Every garage conversion in Long Beach triggers the same fire-safety requirements: a fire-rated separation between the unit and the main house, a self-closing solid-core door, and a hardwired smoke and carbon monoxide system. These aren’t optional extras and they aren’t where you want to save money. A real building professional will flag the danger and build it right, no shortcuts.

How Long Does a Junior ADU Conversion Take?

Most JADU and interior conversions in Long Beach take 3 to 6 months from design to final inspection. Plan check typically runs 3 to 6 weeks, construction runs 8 to 14 weeks for a JADU, and closeout with the city takes another 1 to 3 weeks. Attached ADU conversions with full kitchens and baths run 4 to 7 months end to end.

Prefab interior components compress the construction phase. When the kitchenette, bathroom module, and wall panels arrive pre-finished, the on-site work becomes connect-and-finish rather than build-from-scratch. Our Long Beach clients using Abodu, Boxabl, or Dvele interior kits regularly see the construction phase drop from 12 weeks to 7 or 8 weeks. That’s real time back on the rental clock.

FAQs – Junior ADU & Interior Conversion Cost

How much does a JADU cost in Long Beach in 2026?

A JADU in Long Beach costs $45,000 to $90,000 for a finished unit up to 500 square feet, including permits, plumbing, electrical, kitchenette, and a 3/4 bathroom. If the space is already partly plumbed or finished, expect the lower end. If you’re starting with a bare garage, expect the upper end. Call (562) 362-3885 for a written quote on your specific space.

Is it cheaper to convert a garage or build a new ADU from scratch?

Converting a garage is almost always cheaper, at $85,000 to $150,000 in Long Beach versus $250,000 to $450,000 for a new detached ADU. The conversion reuses the foundation, walls, and roof already on your property. The tradeoff is square footage and ceiling height, which are fixed by the existing structure. If your garage is a standard two-car with good bones, conversion wins on almost every budget.

Do I need permits for an interior ADU conversion in Long Beach?

Yes. A legal JADU or ADU in Long Beach requires building, electrical, plumbing, and mechanical permits, plus plan check through the city’s building department. Permit fees run $4,000 to $9,000. An unpermitted conversion won’t add appraised value, can block a future sale, and puts you at risk with your homeowners insurance. Do it permitted, do it once.

How much does it cost to add a bathroom and kitchenette to a JADU?

In Long Beach, a 3/4 bathroom addition costs $12,000 to $22,000 and a kitchenette costs $8,000 to $18,000, depending on finishes and how far the new plumbing runs from existing lines. A full kitchen with a range, vent hood, and full-size refrigerator runs $18,000 to $40,000. Prefab modules from Dvele or Mighty Buildings can reduce labor costs on both.

Can I do the conversion myself to save money?

You can act as an owner-builder in Long Beach, but most homeowners don’t save what they expect once they pay for plans, permits, subcontractor coordination, and corrections on failed inspections. Electrical and plumbing work must be done by licensed tradespeople. And when something goes wrong, there’s no written guarantee to fall back on. We recommend getting a written price from a contractor, then deciding with real numbers.

Why Long Beach Homeowners Choose Written Pricing

Long Beach has a lot of older housing stock, a lot of coastal moisture, and a lot of contractors. The homes are full of surprises behind the walls, and surprises are where verbal estimates go to die. A contractor who gives you a spoken number and a handshake hasn’t priced your project. They’ve priced a version of your project that doesn’t exist yet.

Benson ADU Long Beach is part of the Relay Home Co. family, established 2006. Every project we build in Long Beach is governed by The Relay Visibility Guarantee: a written price before work starts, real-time tracking during the build, and a 365-Day Done Right Promise. If work isn’t right within a year, we come back and make it right, backed by a written guarantee on every project. Background-checked, uniformed technicians on every job site, and a live person answers the phone around the clock, every day of the year.

Troy Eastman leads our Long Beach team, and the standard he holds is simple: no homeowner should ever wonder what the final invoice will say. The price you’re quoted is the price you pay, in writing, before a single crew member walks onto your property.

We build with Abodu, Cover, Dvele, Mighty Buildings, Boxabl, Plant Prefab, and Tuff Shed, plus James Hardie cladding and trim. That gives you researched, name-brand options instead of one take-it-or-leave-it product. You can look up every one of those brands yourself. We’d rather you do.

If you’re ready to get a real number for your JADU or interior conversion, start with a free estimate. Call (562) 362-3885 and you’ll reach a live person, not a voicemail tree. You’ll get a written price, a straight answer on what’s feasible in your space, and a clear path forward. Or start with our home page to see how we approach ADU construction differently.

Ready to Price Your Long Beach JADU?

You now know the ranges, the cost drivers, and the questions to ask. The next step is a written number for your specific house. Benson ADU Long Beach offers free estimates with no obligation and no pressure. Call (562) 362-3885 today, and you’ll get a written price before any work starts. That’s how it should work.

Pricing reflects the Long Beach market as of 2026. Benson ADU Long Beach offers free estimates – call (562) 362-3885.

Written by Troy Eastman, Owner at Benson ADU Long Beach, serving Long Beach since 2006.