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 · Drawings from $997 (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
11 years in California residential permitting and ADU documentation. This guide was researched and verified against Longmont's building department records and current 2026 California ADU law.
Property Eligibility
Is My Longmont, CO Property Eligible for an ADU?
Longmont homeowners benefit from California's state ADU law — one of the most permissive frameworks in the country since 2020. Here's what applies to your property before you start designing.
Single-Family Home
Permitted on single-family lots in Longmont under California state law. No owner-occupancy requirement in Longmont — you can rent both units as investment properties. Ministerial permit approval — no neighbor input or discretionary design review.
Multi-Family Property
Up to 2 detached ADUs plus JADUs (Junior ADUs) within the existing main structure are permitted under California state law. Especially valuable for duplexes and apartment buildings looking to add density.
HOA-Governed Properties
Approximately 46% of Longmont homes are in an HOA. California law (AB 670) prohibits HOAs from banning ADUs outright, but they can impose reasonable architectural standards. Our drawings include HOA submission formatting.
Historic Districts
Yes - in historic districts only
Always verify eligibility with Building Services Department before designing.
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. Assumes ~85% occupancy, ~30% expense ratio, 23% property value uplift. Consult a financial advisor before investing.
Data sources: Longmont Development Services Department, Zillow Rental Market Reports, local appraiser records. Last verified: 2026-05-30.
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
Longmont ADU Zoning Regulations
| Regulation | Longmont Allowance |
|---|---|
| Max ADU Size | 800 sq ft |
| Max Height | 35 ft |
| Side Setback | 5 ft |
| Rear Setback | 5 ft |
| Owner Occupancy | Not required |
| Short-Term Rental | Permitted |
| Coastal Overlay | No |
| Design Review | Yes - in historic districts only |
| HOA Prevalence | 46% of properties |
Data verified recently. 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 California 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
$997 – $1,997
- ✓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
Transparent Pricing
Longmont ADU Drawing Costs — Compared
What homeowners typically pay for permit-ready ADU drawings in Longmont, by service type:
| Service Type | Price Range | Turnaround | Customization |
|---|---|---|---|
| Local Architect | $3,000 – $8,000 | 4–8 weeks | Fully custom |
| Build With A Plan · Starter | $997 | 10 days | Standard |
| Build With A Plan · Standard | $1,497 | 10 days | Standard |
| Build With A Plan · Premium | $1,997 | 7 days | Enhanced |
- ✓Site plan
- ✓Floor plans
- ✓All 4 elevations
- ✓Electrical plan
- ✓Title 24 energy 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 7-day turnaround
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 real project outcomes from clients who used our permit-ready drawing sets across California.
"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."
Client first names and last initials only. Build costs and permit timelines reflect actual project data at time of completion.
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
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 included in the permit set? +
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
Start Your Longmont ADU Today
Permit-ready drawings from $997. Delivered in 10 business days. Designed for Longmont's building department. 14-day money-back guarantee.
Get My ADU Drawings →Questions? Your coordinator responds within 4 business hours.