Trusted Junior ADU & Interior Conversion for Simi Valley Homeowners
Our Junior ADU & Interior Conversion in Simi Valley typically runs $85,000-$175,000 all-in, drawings through final inspection, depending on whether you’re converting existing square footage or building an attached addition. Most Simi Valley projects move from signed contract to permit submission in 4-6 weeks, with construction lasting 3-5 months for interior work and 4-6 months for attached additions. We’re Ellery ADU Studio, a design-build practice led by Nadia Ellery, and we’ve designed, permitted, and built over 900 ADUs since 2015 - every one at a written price agreed to before any work started.
Simi Valley homeowners who bought between 2015 and 2025 are sitting on some of the highest cost-basis properties in Ventura County. A Junior ADU - legally capped at 500 square feet and contained entirely within your existing home’s footprint - lets you generate rental income or house family without the land-use battles that stall detached units. Our home page outlines our full service range, but this page is specific to interior conversions and Junior ADUs in Simi Valley: basement ADUs, above-garage units, room conversions, attached additions, and Interior JADUs under California’s owner-occupancy rules.
We operate under The Haven Standard. Clause 1 requires a written price before any work begins. Clause 5 locks the scope at signing - no door-side upsells, no scope creep. Clause 7 is our 365-Day Done Right Promise: if the finished ADU isn’t built to the permitted drawings, we make it right, in writing, before the first nail goes in. You can reach us at (820) 444-6584. A live person answers 24/7.
What Our Junior ADU & Interior Conversion Service Includes
Basement ADU
A basement ADU converts your below-grade square footage into a self-contained dwelling unit with its own kitchen, bathroom, sleeping area, and separate exterior entrance. In Simi Valley, this makes particular sense for homes in the Tierra Rejada Valley and north-side neighborhoods where basements are common and hillside grading already created the space. We see a lot of 1970s-1990s homes in the Wood Ranch area with 7-8 foot basement clearances that need only egress window installation and moisture management to become code-compliant.
When you need it: you’ve got 600-1,200 square feet of unfinished basement that’s storing holiday decorations and losing value to dampness. The conversion stops the decay and starts the cash flow.
What Ellery ADU Studio Simi Valley does specifically: we run a full moisture and radon assessment before drawing anything, because California’s HCD code has specific ventilation requirements for basement dwelling units. Our permit set includes the egress window schedule, the radon mitigation plan, and the fire-separation assembly between basement and main house - submitted as one package, built under one contract. No architect blaming the contractor for missed code items, no contractor claiming the drawings were unclear.
Above-Garage ADU
An above-garage ADU adds a second story over your existing garage structure, creating 400-500 square feet of new dwelling space without reducing your yard. This works in Simi Valley’s flatter neighborhoods like Sinaloa Lake and the older tracts near First Street where garages are detached and rooflines are simple. The structural question is always the same: can the existing garage foundation and walls carry a second story, or does the whole structure need replacement?
When you need it: your garage is structurally sound but underutilized, and you want rental income without sacrificing backyard space for a detached unit. In Simi Valley’s hillsides, flat lots with detached garages are prime candidates.
What Ellery ADU Studio Simi Valley does specifically: we bring a structural engineer to the first site visit, not after the drawings are done. If the foundation needs replacement, that’s in your written price from day one. We coordinate the structural calcs, the Title 24 energy compliance, and the fire-rated separation between garage and living space - all within the same contract. For prefab structural systems, we specify and coordinate Cover and Dvele modules when their panelized approach saves time on above-garage framing.
Room-to-ADU Conversion
A room-to-ADU conversion takes existing living space - typically a master suite, a large bonus room, or a ground-floor bedroom wing - and separates it into a Junior ADU with kitchenette, bathroom, and private entrance. In Simi Valley’s 1980s-2000s tract homes, we regularly see 400-500 square foot master suites in the Royal Heights and Indian Hills areas that convert cleanly with one new exterior door and a kitchenette wall.
When you need it: you’ve got adult children returning, aging parents who need proximity, or you want rental income from space you’re not using. The room-to-ADU path is fastest because the envelope already exists.
What Ellery ADU Studio Simi Valley does specifically: we verify the room’s square footage against your lot’s maximum allowable JADU size before drawing anything. Simi Valley’s municipal code follows California’s statewide standards, but your specific zone may have additional requirements. Our permit set includes the fire-rated door assembly between JADU and main house, the separate meter or sub-meter configuration, and the owner-occupancy affidavit - because a JADU requires you to live in either the main house or the JADU.
Attached Addition ADU
An attached addition ADU extends your existing home’s footprint with new construction that shares at least one wall with the main house. Unlike a JADU, it’s not limited to 500 square feet, and unlike a detached ADU, it doesn’t trigger the same setback requirements. In Simi Valley, this suits homeowners on corner lots in the Santa Susana Knolls area or along the wider lots on Cochran Street where side-yard setbacks allow modest extensions.
When you need it: you need more than 500 square feet, or your interior layout doesn’t allow a clean separation, or you want the ADU to read as part of the original architecture for resale value.
What Ellery ADU Studio Simi Valley does specifically: we model the addition in 3D against your existing roofline before you sign, so you see how the massing reads from the street. Our permit set includes the structural connection detail between existing and new construction, the Title 24 compliance package, and the fire-separation rating - all drawn to the city’s exact format requirements. We’ve submitted over 200 attached addition permits in Ventura County since 2015; we know which plan checkers flag which details.
Interior JADU
An Interior JADU - Junior Accessory Dwelling Unit - is the most restrictive and most affordable conversion type: maximum 500 square feet, contained entirely within the existing walls, with efficiency kitchen and separate entrance. California law allows one JADU per single-family lot in addition to a detached ADU, making it a powerful tool for Simi Valley homeowners who want two rental streams. The catch: owner occupancy is mandatory, and the JADU cannot be sold separately.
When you need it: you want the fastest, lowest-cost path to legal rental income, and you’re committed to living on the property. Typical Simi Valley JADUs rent for $1,400-$1,900 monthly.
What Ellery ADU Studio Simi Valley does specifically: we audit your floor plan against the 500-square-foot cap before any design work, because exceeding it by even one square foot disqualifies the JADU path. Our permit set includes the efficiency kitchen layout meeting HCD’s minimum requirements, the separate exterior door with landing and stair compliance, and the owner-occupancy covenant recorded against title. We also flag the utility separation options: separate electric meter, sub-meter, or written shared-utility agreement - your choice, priced upfront.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Brands We Service for Junior ADU & Interior Conversion
Our design-build model includes specifying and coordinating prefab and modular systems when they speed construction or improve quality control. For interior conversions and Junior ADUs, we regularly integrate Cover’s panelized wall systems for above-garage additions where site access is tight and weather protection matters. Their insulated panels arrive with electrical chases pre-cut, which reduces the rough-in schedule by a week on a typical Simi Valley above-garage project. We’ve also specified Dvele’s modular bathroom pods for basement ADUs where below-grade moisture makes site-built tile work risky - the pod arrives fully finished, pressure-tested, and warrantied as a unit.
For structural components in attached additions, we coordinate Mighty Buildings’ 3D-printed wall panels when the geometry is complex and traditional framing would generate excessive waste. And for homeowners exploring the most compact JADU kitchenettes, Boxabl’s Casita modules have influenced our space-planning standards - though we typically adapt their layouts to site-built construction rather than dropping in a complete module. Whether your project involves these brands or conventional stick-frame construction, we specify, coordinate, and warrant the complete installation under one written contract. Our Junior ADU & Interior Conversion in Moorpark page covers similar terrain in a neighboring market with slightly different permit timing.
Signs You Need Junior ADU & Interior Conversion Right Now
- Your mortgage payment has outpaced your income growth. Simi Valley homeowners who bought at 2021-2022 peaks are facing the highest debt-to-income ratios in a decade. A JADU generating $1,400-$1,900 monthly shifts your property from cost center to income producer. The longer you wait, the more months of lost rental income you absorb.
- Your adult children have moved back, or your parents need proximity. Multi-generational living isn’t temporary anymore in Ventura County. A room-to-ADU conversion gives everyone privacy and a kitchenette without the $400,000+ cost of a detached unit. The alternative - years of shared bathrooms and compromised boundaries - degrades family relationships and property function.
- Your basement smells musty every spring. Simi Valley’s winter rains and summer dry cycles create expansion and contraction in hillside basement walls. That musty smell is moisture penetration, and it’s doing structural damage whether you finish the basement or not. Converting to a code-compliant ADU forces the moisture remediation that protects your foundation.
- Your garage is structurally sound but functionally obsolete. Single-car garages from 1960s-1970s Simi Valley tracts don’t fit modern vehicles. The foundation and framing often have decades of life, but the space is too narrow for use. An above-garage ADU leverages that sunk cost into new square footage without the land-use battles of a backyard unit.
- You’re considering selling within 5-10 years. A permitted, inspected ADU adds documented square footage to your assessable improvements. Unpermitted “in-law suites” trigger disclosure problems, appraisal discounts, and buyer walkaways. The permit set we produce becomes a transferable asset that survives the sale.
Our Junior ADU & Interior Conversion Process - Step by Step
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Site Assessment & Feasibility Study. We visit your Simi Valley property with a checklist derived from 900+ previous ADU projects. We measure existing square footage, photograph structural conditions, and run preliminary code checks against your zoning. You receive a documented photo record of what we found, not a summary of how we felt about it. Within 48 hours, you know whether your project is a JADU, attached addition, or room conversion - and why.
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Written Price & Scope Lock. Under Haven Standard Clause 1, you receive a line-item written price before any design work begins. The price includes the permit set, the construction, and the final inspection coordination. Under Clause 5, that scope locks at signing. If we missed something, we absorb it. If you change something, we document the change order in writing with its exact cost. No door-side upsells.
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Permit Set Production. Our architects and structural engineers produce the complete submittal package: floor plans, elevations, structural calcs, Title 24 energy compliance, and all Simi Valley-specific forms. This is the core product, not an afterthought. We’ve been refining our permit sets since 2015, the year California’s ADU laws shifted, and we know which details trigger plan-checker questions. We answer them before submission.
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Construction & Documented Progress. Our background-checked, uniformed crews build to the permitted drawings. Every site visit generates a documented photo record that you receive - evidence of what was found and what was done. If the existing slab is 4 inches, within range for a conversion, we show you the measurement and explain what that means for your foundation line item. If we hit unexpected conditions, we photograph them and call before proceeding.
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Final Inspection & 365-Day Done Right Promise. We coordinate all inspections, pass them, and deliver your certificate of occupancy. Then Clause 7 activates: for 365 days, if anything in the finished ADU is not built to the permitted drawings, we make it right. The promise is in writing, signed before construction starts, not a verbal assurance after the check clears.
How Much Does Junior ADU & Interior Conversion Cost in Simi Valley?
How much Junior ADU & Interior Conversion costs in Simi Valley depends on your starting point. Here’s what we see in our 900+ projects:
| Project Type | Typical Range | Key Variables |
|---|---|---|
| Interior JADU (room conversion) | $85,000-$125,000 | Kitchenette size, bathroom proximity, separate entrance difficulty |
| Basement ADU | $110,000-$165,000 | Ceiling height, egress window count, moisture remediation extent |
| Above-Garage ADU | $130,000-$175,000 | Structural capacity of existing garage, stair access, utility routing |
| Room-to-ADU Conversion | $90,000-$135,000 | Existing bathroom sharing vs. new bathroom, kitchenette complexity |
| Attached Addition ADU | $140,000-$220,000 | Foundation type, roof tie-in complexity, square footage |
What moves you within these ranges? Three factors dominate Simi Valley projects. First, utility separation - running new electric service or plumbing laterals adds $8,000-$15,000 that shared-utility agreements avoid. Second, existing conditions: a basement with 8-foot ceilings and no moisture issues sits at the low end; one with 7-foot ceilings and active seepage moves toward the high end. Third, finish level: we quote mid-grade finishes as standard; upgrades to quartz, hardwood, or custom tile are change-ordered in writing if you select them.
How to avoid overpaying: get the permit set quality right. A cheap permit set that misses Simi Valley’s specific fire-separation detail or HCD’s ventilation requirement generates plan-check revisions that cost weeks and thousands. We include one complete permit resubmission in our base price if the city requests changes - but our resubmission rate is under 5% because we draw it correctly the first time.
What’s included in our free estimate: site visit, feasibility study, documented photo record, and written line-item price. Not included: geotechnical work for hillside properties, which we flag upfront if needed, or HOA architectural review fees, which vary by Simi Valley neighborhood. Call (820) 444-6584 to schedule. Estimates are free, and we offer a Free Second Opinion on any written estimate already in your hand.
Junior ADU & Interior Conversion Near Simi Valley - Our Service Area
We design, permit, and build Junior ADUs and interior conversions throughout Ventura County and the western San Fernando Valley. Our typical site assessment response in Simi Valley is 24-48 hours, with Moorpark and Thousand Oaks running similar. For Junior ADU & Interior Conversion in West Hills, Agoura Hills, Calabasas, and Chatsworth, we schedule assessments within 72 hours. Canoga Park, Winnetka, Northridge, Tarzana, and Reseda fall within our Los Angeles County service zone, with slightly longer permit coordination timelines due to LADBS processing. Every project, regardless of city, operates under the same Haven Standard and the same written-price guarantee.
Serving Simi Valley, CA - Our Local Coverage Area
We’re based in the Simi Valley area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
Frequently Asked Questions - Junior ADU & Interior Conversion in Simi Valley
A Junior ADU is a dwelling unit of 500 square feet or less contained entirely within an existing single-family home, with an efficiency kitchen, separate entrance, and owner-occupancy requirement. Interior conversion is the broader category that includes JADUs, room-to-ADU conversions, basement ADUs, and above-garage units - all using existing structure rather than new detached construction. At Ellery ADU Studio, we handle the design, permitting, and construction of all interior conversion types under one contract at one written price.
Most Simi Valley interior conversions move from signed contract to permit submission in 4-6 weeks, with construction lasting 3-5 months for work within the existing envelope and 4-6 months for attached additions requiring foundation work. Basement ADUs often run longer if moisture remediation or egress window installation hits bedrock. We include a written schedule with every estimate, and Haven Standard Clause 5 prevents scope changes that would extend it without your written approval. Call (820) 444-6584 for a timeline specific to your property.
A typical Interior JADU in Simi Valley runs $85,000-$125,000 all-in, from feasibility study through final inspection, with room-to-ADU conversions at the lower end and basement or above-garage work at the higher end. Attached addition ADUs, which are not Junior ADUs but fall under our interior conversion service, typically run $140,000-$220,000. Every project receives a written line-item price before any work begins, per Haven Standard Clause 1. Call (820) 444-6584 for an exact quote - estimates are free.
Yes. We specify and coordinate Cover’s panelized wall systems for above-garage additions, Dvele’s modular bathroom pods for basement moisture-sensitive applications, and Mighty Buildings’ 3D-printed panels for complex attached addition geometries. Boxabl’s compact layouts inform our JADU space planning. All prefab coordination is included in our single-contract, single-written-price model. If you’re considering a prefab component, we’ll price it against conventional construction so you can compare.
Interior conversions are planned construction, not emergency services, so we do not promise same-day dispatch or 24/7 construction crews. However, a live person answers our phones 24/7 at (820) 444-6584, and we can schedule urgent assessments for situations like post-storm basement flooding that threatens an in-progress conversion or structural concerns discovered during demolition. For true emergencies involving active water intrusion or structural instability, we coordinate immediate stabilization before resuming the conversion schedule.
Our 365-Day Done Right Promise, Haven Standard Clause 7, covers any deviation between the finished construction and the permitted drawings for one full year after certificate of occupancy. This is in addition to manufacturer warranties on fixtures, appliances, and prefab components we specify. The promise is signed in writing before construction starts, not offered as a verbal afterthought. If it’s not right, we make it right.
Gather your property’s legal description or APN, any existing floor plans or surveys, and your rough idea of budget and timeline. If you have a written estimate from another firm, bring it - we offer a Free Second Opinion that identifies what’s included, what’s omitted, and where the risks sit. We’ll photograph existing conditions during our site visit, so you don’t need to document anything yourself. Call (820) 444-6584 to schedule; a live person answers 24/7.
Schedule Your Junior ADU & Interior Conversion Service in Simi Valley Today
Call (820) 444-6584 to schedule your free estimate. You’ll receive a site visit, documented photo record, feasibility study, and written line-item price - all before any design work begins, per Haven Standard Clause 1. Nadia Ellery and our team have designed, permitted, and built over 900 ADUs since 2015. Every one started with a written price and ended with a documented result. Your Simi Valley property is next.
Written by Nadia Ellery, Owner at Ellery ADU Studio, serving Simi Valley since 2015.