ADU Case Studies — Real Project Costs | Cornerstone Capital Development Services

CCDS Project Case Studies — Inland Empire & San Gabriel Valley

Two ADUs. Two Financing Paths.
Real Costs from Real Projects.

An attached ADU in Ontario with three scope overruns — and a city-funded garage conversion in Pomona currently moving through permits. Here is what the numbers actually look like.

Case Study 1
Ontario — 573 Sq. Ft. Attached ADU
Inland Empire  ·  Cash-out refinance  ·  $171,850 total
Completed
Case Study 2
Pomona — 400 Sq. Ft. Garage Conversion
San Gabriel Valley  ·  City ADU loan  ·  $127,000 total
In Permitting
Case Study 1 — Ontario, California

573 Sq. Ft. Attached ADU — $139,850 Base / $171,850 Final

Attached ADU  ·  Cash-out refinance  ·  One cost-saving decision + two scope overruns totaling $32,000  ·  $2,050/month rental revenue

Completed — Occupied

This homeowner in Ontario, CA added a 573 sq. ft. attached ADU to their existing single-family residence, designed for long-term family occupancy. The unit includes a full kitchen, private bathroom, sleeping area, and in-unit laundry. CCDS served as licensed contractor and land use consultant throughout, from Phase 1 feasibility — where a water meter cost comparison identified $7,150 in savings — through final Certificate of Occupancy.

Base contract
$139,850
Overruns
+$32,000
Total cost
$171,850
$/sq. ft.
$300
Monthly rent
$2,050
Break-even
~7 years

Full cost breakdown

Line item Amount
Base construction contract$139,850
Cost Decision 1 — Owner-selected water sub-meter (vs. $11,500 street meter alternative — saves $7,150) $4,350
Overrun 2 — New laundry room and appliances $6,300
Overrun 3 — Perimeter site work (patio, sidewalk, fencing) $21,350
Total above base contract — includes $4,350 cost decision + $27,650 scope overruns (22.9% above base) $32,000
Total project cost $171,850

The three overruns — what happened and what to do instead

Cost Decision 01
$4,350

Owner-selected water sub-meter — saves $7,150

Two metering options were evaluated during Phase 1. Option A: a new street meter at an estimated $11,500 — including City of Ontario permit fees, water district connection fees, right-of-way trenching, and a Civil 'A' contractor for traffic control. Option B: an interior sub-meter on the existing service at $4,350. The homeowner chose Option B, saving $7,150.

Key takeaway: This was a deliberate, cost-effective decision — not an unforeseen requirement. Early Phase 1 utility evaluation is what made the comparison possible. Always request both metering options and their full costs from your utility district before committing to design.
Overrun 02
$6,300

New laundry room and appliances

Dedicated in-unit laundry added after permits were issued. Required a formal plan revision, additional plumbing and electrical rough-in, and appliance costs at mid-construction pricing — totaling $6,300.

Fix: In-unit laundry is a top tenant priority. Design it in from the start — post-permit plan revisions cost significantly more than original scope.
Overrun 03
$21,350

Perimeter site work — patio, sidewalk, fencing

Patio, sidewalk, and fencing added post-permit to define the ADU's private outdoor area — the largest single overrun at $21,350. Owner-directed additions that felt secondary until construction was already underway.

Fix: At $21,350 this was the costliest decision made after contract signing. Walk the full property before signing and price all exterior site improvements into the original contract.
The contingency rule — illustrated A 22.9% total variance is above the recommended 10–20% contingency buffer — though $4,350 of that figure reflects a deliberate cost-saving decision (sub-meter vs. $11,500 street meter). The remaining $27,650 represents two avoidable scope additions made after the contract was signed. The largest single item — $21,350 in perimeter site work — is the most important lesson: what feels secondary in the planning stage becomes the largest line item on the final invoice. Price it before you sign.

Investment return

Total invested
$171,850
Cost / sq. ft.
$300
Monthly rental
$2,050
Annual revenue
$24,600
Break-even
~7 yrs
Financing
Cash-out refi

Break-even reflects full construction cost recovery from rental income only. Does not include property value appreciation or financing interest costs.

"Phase 1 utility evaluation on this project saved the homeowner $7,150 — the difference between a $4,350 sub-meter and an $11,500 street meter with right-of-way permits and district fees. That's the value of doing the utility comparison early. The $21,350 in site work is the other lesson: the fence, the patio, the sidewalk — those decisions did not get cheaper once the crew was on site."

— CCDS Field Team, Ontario Project
Case Study 2 — City of Pomona, California

400 Sq. Ft. Garage Conversion ADU — $127,000 City-Funded Loan

1-bedroom garage conversion  ·  City of Pomona low-interest ADU loan  ·  Currently in permitting  ·  Anticipated $1,200–$1,500/month

In Permitting — Construction Pending

This homeowner in the City of Pomona is converting an existing garage into a 400 sq. ft. one-bedroom ADU, funded entirely through the City of Pomona's low-interest ADU Loan Program — a municipally administered financing vehicle designed to help qualifying homeowners develop ADUs without conventional construction lending. The project is currently in the permitting phase; construction has not yet begun.

ADU type
Garage conv.
Size
400 sq. ft.
Bedrooms
1 BR
Construction loan
$120,000
Plans loan
$7,000
Total funding
$127,000

Full financing breakdown

Funding component Source Amount
Architectural plansCity of Pomona Low-Interest ADU Loan$7,000
Construction costsCity of Pomona Low-Interest ADU Loan$120,000
Total project funding$127,000

The City of Pomona ADU Loan Program

The City of Pomona administers a low-interest ADU loan program as part of its Housing Element compliance strategy — making below-market-rate construction financing available to qualifying homeowners who may not qualify for conventional lending or who want to avoid resetting their existing mortgage.

This program is a compelling alternative to cash-out refinancing or HELOC financing for three reasons:

  • Interest rates are set below conventional market rates by the city
  • Plans costs ($7,000) are included in the loan — covering pre-construction expenses that often must be paid out-of-pocket with conventional financing
  • The homeowner's existing mortgage is preserved untouched
CCDS contractor note — know your municipal programs before choosing a financing path Several Inland Empire and San Gabriel Valley cities — including Pomona — operate ADU loan or grant programs as part of their Housing Element compliance strategies. These programs are frequently underutilized because homeowners don't know they exist. CCDS conducts a local program review as part of every Phase 1 feasibility analysis to identify available funding before recommending a financing path. This single step can eliminate the need for conventional lending entirely.

Why a garage conversion

Garage conversions are among the most cost-efficient ADU types in California. The existing foundation, roof structure, and utility proximity significantly reduce construction costs compared to a detached or attached new-build ADU. California law (AB 2221 and SB 897) specifically protects garage conversions from many local restrictions — prohibiting replacement parking requirements and limiting setback rules.

For this Pomona project, a 400 sq. ft. one-bedroom unit is well-calibrated for the city's rental market, which supports single occupants and couples at competitive rents. The $127,000 total budget — including plans — aligns with Inland Empire / San Gabriel Valley market rates for a conversion of this scope.

Anticipated investment return

Total loan
$127,000
ADU type
Garage conv.
Size
400 sq. ft.
Anticipated rent
$1,200–$1,500
Mid-point estimate
$1,350/mo
Est. break-even
~7–9 years
Note on projected figures Because this project is currently in permitting, rental revenue is anticipated — not confirmed. Break-even estimates assume occupancy at the mid-point of the anticipated rental range ($1,350/month) with no vacancy adjustment applied. CCDS will update this case study with actual rental and construction cost data once the project reaches completion and occupancy.

"Homeowners who engage a land use consultant at the Phase 1 stage — before committing to any financing path — can access municipal programs that fundamentally change the project economics. The Pomona homeowner avoided conventional lending entirely. That's the value of doing the program research before signing anything."

— CCDS Field Team, Pomona Project

Property profile — visual reference

Exhibits A–E: Understanding Your Property Before You Build

Before committing to any ADU design or contract, understanding four characteristics of your property is essential: lot size, shape and configuration, slope and topography, and orientation. Exhibits A through D illustrate each. Exhibit E provides a sample site plan showing how setback distances apply to an attached ADU on a standard interior lot.

Exhibit A — Lot Size
Diagram showing lot lines and property boundaries for ADU planning — lot size illustration
Lot lines define the legal boundaries and total lot area — used to determine setbacks, lot coverage limits, and which ADU types are permissible.

Field Guide Section 5: Lot size under 3,500 sq. ft. → JADU or conversion. 3,500–5,000 sq. ft. → attached or small detached. 5,000–10,000 sq. ft. → full-size detached ADU.

Exhibit B — Shape and Configuration
Diagram showing rectangular, pie, reverse pie, and irregular lot shapes for ADU planning
Lot shape — rectangular, pie, reverse pie, or irregular — affects where an ADU can be placed and how it can be accessed from the street.

Field Guide Section 5: Irregular and pie-shaped lots may constrain ADU placement due to setback geometry. Identify shape before committing to a design.

Exhibit C — Slope and Topography
Diagram illustrating sloped lot topography and its impact on ADU foundation and grading costs
A sloping lot increases foundation cost and drainage complexity. Significant grade change may require retaining walls, specialized foundations, or additional grading — all affecting budget and permitting.

Field Guide Section 5: Evaluate slope conditions during Phase 1 feasibility before committing to a design or signing a contract.

Exhibit D — Lot Orientation
Diagram showing corner, interior, through, flag, and condominium lot orientations for ADU setback analysis
Orientation describes how your parcel relates to surrounding streets. Corner, interior, through, flag, and condominium lots each carry distinct setback rules and ADU access implications.

Field Guide Section 5: Orientation affects setbacks, privacy, natural light, and the most practical location for a separate ADU entrance.

Exhibit E — Site Plan — Sample / Interior Lot
Sample site plan for an attached ADU on a standard interior lot showing front yard, side yard, rear yard, and setback distances
Sample site plan for an attached ADU on a standard interior lot, showing front yard setback, side yard setback, rear yard setback, and ADU setback distances from all property lines.

Field Guide Section 3 (What is an ADU?) and Section 5 (Planning and Due Diligence). The Ontario Case Study 1 follows this interior lot / attached ADU configuration.

Note on exhibit illustrations Exhibits A–E are property profile diagrams embedded in Field Guide v1.1. The full-resolution diagrams are included in the CCDS ADU Field Guide (Sections 3 and 5). Contact CCDS to request the guide or schedule a feasibility review that walks through your specific property profile.

Side-by-side

Both Projects Compared

Two very different approaches to ADU development — same market region, different ADU types, and completely different financing paths.

Case Study 1 — Ontario
ADU Type
Attached ADU — new construction
Size
573 sq. ft.
Bedrooms
Not specified (full kitchen, bath, laundry)
Base contract
$139,850
Cost overruns
$32,000 total — $4,350 cost decision + $27,650 scope overruns
Total cost
$171,850
Cost / sq. ft.
$300
Financing
Cash-out refinance (conventional)
Monthly rental
$2,050 (confirmed)
Break-even
~7 years
Status
Completed & occupied
Case Study 2 — Pomona
ADU Type
Garage conversion ADU
Size
400 sq. ft.
Bedrooms
1 bedroom
Plans cost
$7,000 (City loan)
Cost overruns
None anticipated (in permitting)
Total funding
$127,000
Cost / sq. ft.
~$300 (projected)
Financing
City of Pomona low-interest ADU loan
Monthly rental
$1,200–$1,500 (anticipated)
Break-even
~7–9 years (projected)
Status
In permitting — construction pending
Key takeaway from the comparison The Ontario project illustrates two distinct lessons: Phase 1 utility evaluation saved $7,150 on water metering, while the $21,350 perimeter site work overrun — added after contract signing — exceeded the entire Pomona plans budget of $7,000. The Pomona project illustrates that a Phase 1 program search — before any financing commitment — can unlock city-funded loan programs that eliminate conventional lending entirely. Both projects demonstrate that $300/sq. ft. is a reliable Inland Empire / San Gabriel Valley benchmark for well-managed ADU construction in 2025–2026.

Financing guide

Four Paths — and How to Choose the Right One

Both case studies illustrate how financing choice shapes total project cost, monthly cash flow, and break-even horizon. Use this framework before committing to any path.

Used — Ontario project

Cash-out refinance

Replaces your existing mortgage with a larger loan; the difference comes to you as cash at closing. Best when your current rate is not substantially below today's market.

  • One monthly payment, no second lien
  • Large lump sum available at closing
  • Resets your mortgage term and rate
  • If existing rate is low, long-term interest rises
Used — Pomona project

Municipal ADU loan program

City-administered low-interest loans for qualifying homeowners. Covers construction and often plans costs. Preserves your existing mortgage entirely.

  • Below-market interest rate set by the city
  • Plans costs often included in loan
  • Existing mortgage rate preserved
  • Income/property eligibility requirements apply
  • Processing timelines tied to city program capacity

HELOC

Revolving credit line against home equity. Best when your existing mortgage rate is below 5% — keep it and borrow as a second lien.

  • Draw in phases; pay interest only on what's used
  • Preserves your existing low mortgage rate
  • Variable rate — payments can increase
  • Post-draw repayment period can spike costs

Construction loan

Short-term loan funding construction draws as milestones are reached, converting to a permanent mortgage. Best for homeowners with limited existing equity.

  • Structured for construction phasing
  • Available without a large equity base
  • Higher rates and fees than other options
  • More documentation and lender oversight

Decision framework

Your situation Recommended path Why
Qualify for a city or county ADU loan program Municipal ADU loan ← Pomona path Below-market rate; plans often included; existing mortgage preserved
Mortgage rate below 5%, significant equity HELOC Protect your low rate; borrow as a second lien
Mortgage rate above 6.5%, significant equity Cash-out refinance ← Ontario path Consolidate at current rates; one payment
Limited equity, strong income Construction loan Access capital without requiring existing equity base
Strong cash reserves, no debt preferred Cash savings Fastest start, zero interest cost
CalHFA ADU Grant — up to $40,000 California homeowners may qualify for up to $40,000 in pre-development and non-recurring closing cost reimbursements through the CalHFA ADU Grant Program, which can be combined with several of the financing paths above. Ask CCDS during your feasibility review whether your project and jurisdiction qualify.

Field Guide v1.1 — what's new

Updated CCDS ADU Field Guide — Version 1.1

Version 1.1 adds substantive updates across every major section of the original guide — driven by real project data, red-line practitioner annotations, competitive analysis, and construction practice refinements. The two case studies are fully incorporated into Section 6, Exhibits A–E are embedded in Sections 3 and 5, and practitioner callouts have been added throughout.

Section What's new Driven by
Section 3 Title renamed from "What an ADU Is" to "What is an ADU?" — Exhibit E (site plan) added as visual reference Title correction + exhibit integration
Section 5 Exhibits A–D added — Lot Size, Shape, Slope/Topography, and Lot Orientation with descriptive captions Exhibit integration
Section 6 Case Study 1 (Ontario) — water meter decision reframed as owner cost-saving choice ($4,350 sub-meter vs. $11,500 street meter, saving $7,150); laundry overrun $6,300; site work overrun $21,350; rental $2,050/mo; break-even ~7 years Ontario project data
Section 6 Case Study 2 (Pomona) — 400 sq. ft. garage conversion, $127K City of Pomona ADU loan, 1BR, in permitting, anticipated $1,200–$1,500/mo Pomona project data
Section 6 Financing Decision Framework — five-path decision table (municipal loan added as first priority) Competitive gap analysis
Section 6A ADU Investment Strategy Narrative — occupancy models, cost control, financial evaluation, exit strategies Updated PDF draft
Section 7A Owner-Builder Risks — legal, insurance, financing, resale, subcontractor exposure and mitigation Updated PDF draft
Sections 7 & 12 Cross-reference callouts linking Ontario project lessons to Phase 1 (water meter cost comparison — $4,350 sub-meter vs. $11,500 street meter) and Phase 4 (plan revision cost impact) Ontario project data
Section 11 Phase 3 permit package expanded — assigned address of proposed ADU and wet-signed, stamped plans by licensed engineer or architect added as required submittal items Permitting practice update
Section 12 Phase 4 first sentence updated — owner-builder exception noted with reference to Section 7A risks; Foundation inspection expanded to include concrete forms, structural connections, hardware, and third-party special inspections; MEP Rough-in expanded to require underground piping and conduit inspection prior to trench backfill Construction practice update
Section 5 Addressing bullet expanded — USPS postmaster coordination added; utility purveyor contact note expanded with processing fees, timelines, and submittal content requirements Red-line annotation
Section 6 New "Grant and Loan Programs" callout — CalHFA pre-construction grants and municipal low-interest loan programs for qualifying homeowners Red-line annotation
Section 9 Best Practices expanded — slope pitch of entries and perimeter hardscape (walks, patios, gutters) for attached/detached ADUs; Budget Alignment Note expanded with plan designer RFI scope requirement; new Surveyor Note callout added Red-line annotation
Section 10 Solar bullet updated — "or proposed" ADUs added; Fire Sprinklers expanded with separate connection and fire district plan review requirements; Dry Utilities expanded with fire district and sprinklers; Schedule Warning expanded with address assignment and development services clerk coordination Red-line annotation
Section 11 Item-by-item resubmittal response requirement added; COA language clarified as mandatory; new Pre-Submittal Coordination callout (GC + designer gap review); contract/agreement dollar amount consistency bullet added; Plan Re-Submittal Requirement callout relocated from Section 12 into Section 11 where it belongs — prior to construction start Red-line annotation
Section 12 Structural Engineer Site Walk added as inspection milestone; engineer site walk bullet updated with written contractor confirmation requirement; Meter Release Warning updated — "final inspection" language clarified. Plan Re-Submittal Requirement callout removed from this section and relocated to Section 11. Red-line annotation
Section 15 Two new Key Takeaways: Property Boundaries (licensed surveyor for staking and future sale) and Plan/Scope of Work Dependencies (concurrent architect-contractor gap resolution) Red-line annotation
Section 17 Watch Manual entry expanded — clarified as ROW work by Civil 'A' contractor; new water meters typically installed within ROW Red-line annotation
Section 14 Checklist updated — water meter cost comparison (both options) and written contingency fund added as explicit items Ontario project lessons

Start with a feasibility review — before financing, before design.

CCDS identifies available city and county ADU loan programs, verifies utility requirements, and builds your project's real cost picture before you commit to any path. Inland Empire and Southern California.

Request Feasibility Review → Download Field Guide v1.1