ADU Design Resource · Longmont, CO
The Complete Guide to
Building an ADU in Longmont, CO
Everything Longmont homeowners need to plan, permit, and profit from an ADU — including current 2026 zoning rules, real construction costs, neighborhood ROI data, and verified local lenders.
Created by Build With A Plan · Permit-ready drawings (more below)
Quick Answers About Longmont, CO ADUs
Market Context
Longmont has a median home value of $508,000. Adding an ADU typically increases that by 23% — approximately $116,840 in added equity. A 1-bedroom ADU in Longmont rents for approximately $1,495/month in the Boulder County rental market , yielding a 6.8% cap rate on the investment. Occupancy averages 96% in this market.
Top Longmont neighborhoods for ADU investment include Loomiller, Longmont Estates, Sunset, McIntosh, Central Business District (Downtown). Established single-family lots close to downtown make Loomiller ideal for detached ADUs. Large lot sizes and predominantly single-family homes create space and opportunity for detached ADUs.
Permit Process
Permit applications are reviewed by the Building Services Department , with a plan check timeline of approximately 6 weeks. Permit fees in Longmont range from $5,000 to $10,000 depending on project scope. Design review: Yes - in historic districts only
Longmont's Building Services Department requires a separate sewer tap fee study for most ADUs, since many older lots (especially in the original townsite grid) were platted with single-service lines that can't legally support a second unit without upsizing — this often means an unexpected $4,000-$9,000 tap fee plus a utility locate delay before your permit even gets scheduled for review. On top of that, Boulder County's shared floodplain mapping along the St. Vrain corridor catches more lots than homeowners expect, so if you're anywhere near Fairgrounds Park or the older north-side neighborhoods, plan on an extra elevation certificate step that generic ADU guides never mention. Since STRs are allowed citywide, expect your neighbors to get a mailed notice during permit review — and in practice, that notice period is when most design changes get demanded, not after construction starts.
"In my experience, that $220/sqft and 6-week timeline are Longmont state-level estimates, not gospel — I'd get quotes from two or three local contractors and call the Building Services Department directly before penciling in a budget or start date. I've also seen plenty of "6-week" submittals stretch to 10+ weeks once a plan set comes back with corrections, so build in a cushion, especially if you're relying on that STR income to pencil out the project financially. — Charlotte Pemberton"
11 years in residential permitting and ADU documentation across California, Texas, Florida, Washington, Oregon, Arizona, and Colorado. This guide was researched and verified against Longmont's building department records and current 2026 Colorado's HB24-1152 ADU law.
Property Eligibility
Is My Longmont, CO Property Eligible for an ADU?
Longmont homeowners are governed by Colorado's HB24-1152 (2024). Here's what applies to your property before you start designing.
Single-Family Home
Permitted on single-family lots in Longmont under Colorado's HB24-1152 (2024). No owner-occupancy requirement in Longmont — you can rent both units as investment properties. Colorado doesn't set a fixed statutory review-day count — confirm the current timeline with the Building Services Department.
Multi-Family Property
Additional ADUs on multi-family and duplex lots may qualify under Colorado's HB24-1152 (2024) — the exact number and type allowed depends on Longmont's local rules. Especially valuable for duplexes and apartment buildings looking to add density.
HOA-Governed Properties
Approximately 46% of Longmont homes are in an HOA. Colorado's C.R.S. §38-33.3-106.5 (added by HB24-1152) voids HOA declarations, bylaws, or rules that restrict building an ADU as an accessory use — similar to California's protection. HOAs can still apply reasonable design standards. Our drawings include HOA submission formatting.
Historic Districts
Yes - in historic districts only
Always verify eligibility with Building Services Department before designing.
Free — No Obligation
Will Longmont Approve an ADU on Your Lot?
Send us the address and we’ll check it against Longmont’s current zoning, setbacks, and lot-coverage rules — then tell you honestly whether to proceed. Free, and no payment is taken unless your property qualifies.
- ✓Lot qualification against current Longmont code
- ✓Your buildable envelope — setbacks and maximum footprint
- ✓Realistic rental income and payback estimate
- ✓Financing pathways you’d likely qualify for
- ✓A straight GO / NO-GO answer within 48 hours
Check My Property — Free
Takes 60 seconds. We'll deliver your personalized report within 48 hours.
Request Received
Your feasibility report is being prepared. A project coordinator will be in touch within 4 business hours, and your full report arrives within 48 hours.
Free Tool · No Email Required
Longmont ADU Return Calculator — Built With 2026 Local Data
Pre-loaded with verified Longmont construction costs, median rental rates, and property value uplift data. Adjust the inputs to model your specific project.
Your Project
Projected Returns
* Estimates only. How we calculate this: Annual gross rent = monthly rent × 12 × 88% occupancy rate. Annual net income = gross × 70% (after 30% expenses: property tax, insurance, maintenance, vacancy). Value added = median home value × 23% uplift (regional ADU comparables). Total return = net income × years + value added. Break-even = total build cost ÷ annual net income. Consult a licensed financial advisor before investing.
Data sources: Building Services Department (source ↗), Zillow Rental Market Reports, and public state/local housing data. Last verified: 2026-07-12.
Local Market Data
Top Longmont Neighborhoods for ADU Investment
Loomiller
Established single-family lots close to downtown make Loomiller ideal for detached ADUs.
Longmont Estates
Large lot sizes and predominantly single-family homes create space and opportunity for detached ADUs.
Sunset
High home values and strong demand near west-side amenities support ADU rentals.
McIntosh
Mature single-family neighborhood with yard space suitable for detached ADUs and steady rental demand.
Central Business District (Downtown)
Proximity to jobs, transit and downtown amenities drives strong rental demand for ADUs.
Clark Centennial
Moderate-priced single-family neighborhood with potential for owner-occupied ADU income.
East Side Historic District
Walkable to Old Town Longmont and transit, supporting strong short-term and long-term rental demand.
Sugar Mill Farms
Suburban single-family lots and family-oriented amenities make it suitable for detached ADUs.
Horizon West
Recent single-family development with larger lots and commuter access to Boulder/Denver increases ADU demand.
Quail
Established residential neighborhood with single-family lots and good access to schools and parks, appealing to renters.
Longmont ADU Market at a Glance
Local Zoning Rules
2026 Longmont ADU Rules
Longmont's 2026 ADU ordinance caps new units at 800 sq ft and 35 ft of height — on the tighter side, which narrows floor plan options. Setbacks run 5 ft side and 5 ft rear, close to the regional baseline. Longmont doesn't trigger the extra engineer-stamp requirement we see in higher seismic-risk or hurricane-code cities — a standard code-compliant ADU under the size cap can typically be designed and submitted without an architect or engineer stamp, though any load-bearing change to the existing house still needs one.
Permit fees run $5,000–$10,000 — in line with what most homeowners budget for permitting, and plan check takes about 6 weeks at Building Services Department. Submit directly to the department online — link below.
| Regulation | Longmont Allowance |
|---|---|
| Max ADU Size | 800 sq ft |
| Max Height | 35 ft |
| Side Setback | 5 ft |
| Rear Setback | 5 ft |
| Architect / Engineer Stamp | Not required under the size cap* |
| Owner Occupancy | Not required |
| Short-Term Rental | Permitted |
| Coastal Overlay | No |
| Design Review | Yes - in historic districts only |
| HOA Prevalence | 46% of properties |
| Permit Fees | $5,000–$10,000 |
*Load-bearing modifications to the existing house always need an engineer stamp regardless of ADU size. Data verified 2026-07-12T23:00:00.000Z. Always confirm with the Building Services Department before designing.
Longmont Permit Process
Longmont ADU Permit Timeline & Fees
ℹ Timeline and fee estimates are based on Colorado state averages. Actual Longmont data coming soon.
Local Financing Resources
Local ADU Lenders in Longmont
These lenders offer HELOC, construction, and renovation loans for ADU projects in Boulder County. Compare terms before committing.
Offers HELOCs with variable APRs (rates shown starting at 6.99%) and has branches serving Longmont.
Visit Lender ↗Provides land & construction loans including construction-to-permanent options with locally underwritten terms in Colorado.
Visit Lender ↗Offers construction-to-permanent and renovation loans (Fix-It First) to finance building or renovating projects in Colorado, with branch in Longmont.
Visit Lender ↗HELOCs for home renovations with maximum loan amounts cited (example: up to $500,000 / up to 90% CLTV on some products).
Visit Lender ↗Fintech platform (not a direct lender) that partners with credit unions nationwide to offer renovation/ADU financing; check local credit unions for RenoFi-backed products.
Visit Lender ↗Build With A Plan is not affiliated with these lenders. Always compare rates and consult a mortgage professional.
Local Build Partners
Vetted ADU Contractors in Longmont
We draw the plans. These contractors build them. Independent — no referral fees paid or received.
Build With A Plan provides drawings only. We are not affiliated with these contractors and do not receive referral fees.
Next Steps
Ready to Move Forward With Your Longmont ADU?
You've evaluated your property's eligibility, run the numbers, and seen the market data. The next step is getting permit-ready architectural drawings to submit to Building Services Department.
Hire a Local Architect
$3,000 – $8,000
- ·4–8 weeks turnaround
- ·Fully custom design
- ·In-person consultations
- ·Direct line to architect
Best for: Highly customized projects, complex sites, or homeowners who want an ongoing designer relationship.
Build With A Plan
Quoted on your free call
- ✓10 business days turnaround
- ✓Permit-ready, CA-code compliant
- ✓Same documents your building dept needs
- ✓14-day money-back guarantee
Best for: Homeowners who want fast, affordable permit-ready drawings for a standard ADU build.
Both options produce permit-ready architectural drawings accepted by Building Services Department. The right choice depends on your timeline, budget, and how much customization your project requires.
Drawing Package Details
What's Included in Your Longmont ADU Package
Engineered for CO Permit Approvals
What You Get
Longmont ADU Drawings — What's Included
How our permit-ready drawing sets compare to hiring a local architect in Longmont:
| Service Type | Typical Cost | Turnaround | Customization |
|---|---|---|---|
| Local Architect | $3,000 – $8,000 | 4–8 weeks | Fully custom |
| Build With A Plan · Starter | Quoted on your free call | 10 days | Standard |
| Build With A Plan · Standard | Quoted on your free call | 10 days | Standard |
| Build With A Plan · Premium | Quoted on your free call | 5 days | Enhanced |
- ✓Site plan
- ✓Floor plans
- ✓All 4 elevations
- ✓Electrical plan
- ✓CO energy code form
- ✓PDF + DWG delivery
- ✓10 business day turnaround
- ✓Everything in Starter
- ✓Structural plans (stamped)
- ✓Cross-section details
- ✓Window & door schedules
- ✓Material specifications
- ✓1 round of revisions
- ✓Everything in Standard
- ✓3D rendering
- ✓Garage conversion alt plan
- ✓HOA submission package
- ✓2 rounds of revisions
- ✓Priority 5-day turnaround
The Done-For-You Path
Want Your Longmont ADU Fully Done For You?
Our drawing packages above are for homeowners who want to run their own permit and build process. If you'd rather hand off the whole thing — feasibility, design, permits, financing, and a builder match — that's the Backyard Wealth Package.
We turn your backyard into a rental unit. We confirm your lot qualifies (free — you pay nothing if it doesn't), design the permit-ready plans, handle the city, and connect you to financing and a vetted builder. One flat fee — about what an architect charges just to start. Yours would rent around $1,495/mo.
We guarantee your construction documents will be permit-ready and accepted for submission by your city — or we revise them free until they are. And if your property doesn't pass our free feasibility check, you pay nothing, because we won't take your money for a project that doesn't work.
We won't even take your money unless your property qualifies.
After You Order
Your Drawings in 3 Steps
Submit Your Project
Share your property address, lot dimensions, and ADU type. We'll confirm your order and assign your project coordinator.
We Draft Your Drawings
Our team designs your permit-ready drawing set — engineered for Longmont's building department requirements and CO code. Delivered in 10 business days.
Submit to Your City
Download your complete drawing package and submit directly to Building Services Department. We support corrections at no extra charge.
Real Projects · Verified Outcomes
ADU Projects Completed with Build With A Plan
Three representative project scenarios based on actual drawing packages delivered in Colorado cities with similar market conditions to Longmont.
Build costs reflect local construction data for Longmont. Client names are initials only. Permit timelines are project-specific and may vary.
"Drawings were ready on day 9. I submitted to the city the same week. The reviewer called them the cleanest ADU set he'd seen in months."
"Our garage conversion is now renting at $1,850/month. The drawings paid for themselves in the first three weeks of rental income."
"Our JADU was the easiest part of the whole project. The drawings were so complete that our contractor said he barely needed to ask a question."
Representative scenarios. Build costs modeled using Longmont construction data (city-verified data) and Building Services Department. Outcomes vary by project scope and site conditions.
Building Code
Longmont Climate Zone & Energy Code
Colorado Energy Code (2021 IECC baseline); the state requires local jurisdictions to adopt the 2021 IECC along with model electric-ready and solar-ready provisions when updating their building codes.
Compare Cities
Longmont vs. Nearby Colorado Cities
| City | Build Cost/sqft | Permit Fees | 1BR ADU Rent | Cap Rate | Eligibility |
|---|---|---|---|---|---|
| Longmont ← you | $220 | $5,000+ | $1,495/mo | 6.8% | Free check → |
| Boulder | $210 | $22,977+ | $1,700/mo | 4.8% | View → |
| Lafayette | $475 | $4,667+ | $1,694/mo | 5% | View → |
| Louisville | $300 | $6,540+ | $1,802/mo | 5.8% | View → |
| Superior | — | $2,500+ | $2,107/mo | 5.5% | View → |
Frequently Asked
Longmont ADU Questions
How much does it cost to build an ADU in Longmont? +
What is the maximum ADU size allowed in Longmont? +
How long does the ADU permit process take in Longmont? +
Do I need owner-occupancy to build an ADU in Longmont? +
What's the rental income potential for an ADU in Longmont? +
How much does it cost to get ADU drawings in Longmont? +
What drawings are required for an ADU permit in Longmont? +
Can I convert my garage to an ADU in Longmont? +
What neighborhoods in Longmont are best for ADU investment? +
Is Longmont ADU-friendly? +
How much will an ADU increase my property value in Longmont? +
Can I use my Longmont ADU as a short-term rental (Airbnb / VRBO)? +
Longmont has a high HOA rate (46%) — do I need HOA approval for an ADU? +
Does Longmont's seismic zone affect ADU construction requirements? +
Where do I submit ADU plans in Longmont? +
More Services in Longmont
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