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ADU / Backyard Cottage in Longmont

Licensed adu / backyard cottage services for Longmont and Boulder County. Build an income-producing unit in your backyard.

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ADU / Backyard Cottage

Build an income-producing unit in your backyard

Local Service

ADU / Backyard Cottage Services in Longmont, Colorado

Autotelic provides professional adu / backyard cottage services throughout Longmont and the surrounding Boulder County area. Accessory dwelling unit construction across the Denver metro — detached new builds, garage conversions, and basement conversions compliant with Colorado HB 24-1152. One veteran-owned design-build team runs the full Denver sequence under a single contract: zoning verification and concept design, municipal zoning and building permit reviews, then construction and inspections through certificate of occupancy. Whether you're in Longmont (north Boulder County) or nearby communities, our team delivers quality workmanship backed by Colorado licensing and full insurance coverage.

Permit Thresholds

Longmont permit thresholds

Code reference
The City of Longmont has adopted the 2024 International Codes, including the International Residential Code, effective July 1, 2026.
Code reference
Retaining walls over 48 inches.
Code reference
Accessory buildings over 120 square feet.
Code reference
Accessory buildings 120 square feet or less in floor area, though it is recommended to secure them against wind damage, and they must still meet setback requirements.
Code reference
Fencing below certain heights (commonly 6 feet).
Foundations
For one-story accessory structures greater than 120 square feet, foundations must be designed by a Colorado-licensed engineer, and the drawings must be stamped and sealed by that engineer.
Code reference
The City of Longmont enforces the 2024 International Codes, which became effective on July 1, 2026, along with the Metro Cohort Model Code.
Code reference
The Longmont Municipal Code incorporates by reference the International Residential Code for One and Two-Family Dwellings, 2021 Edition, including various appendix chapters.

Codes are amended regularly — verify current requirements with **The City of Longmont Building Services (Building Inspection division) issues residential building permits.

Zoning & ADU

Longmont zoning & ADU quick facts

Code reference
The most common single-family residential districts that permit ADUs are Rural Residential (R-RU), Residential-Single Family (R-SF), and Residential-Mixed Neighborhood (R-MN).
Code reference
ADUs are permitted as an accessory use in the R-RU, R-SF, and R-MN zoning districts, among others.
Code reference
General ADU standards are found in Longmont Municipal Code (LMC) Sections 15.04.040 and 15.02.080.E.
Code reference
An ADU cannot exceed one-half (50%) of the finished, above-ground floor area of the main house.
Code reference
An ADU may total up to 800 square feet regardless of the principal dwelling's size.
Code reference
An ADU can be larger than 800 square feet if it does not exceed 50% of the finished, above-ground floor area of the principal dwelling.
Code reference
If an ADU is part of a garage structure, the combined area of the garage and habitable ADU space cannot exceed 800 square feet or 50% of the finished, above-ground floor area of the principal dwelling, whichever is greater.
Code reference
(LMC Sections 15.04.040.C.2 and 15.02.080.E.3)

Codes are amended regularly — verify current requirements with **The City of Longmont Building Services (Building Inspection division) issues residential building permits.

Energy Code

Longmont energy code quick facts

Code reference
2024 IECC (Metro Cohort Model Code based on the 2024 International Energy Conservation Code, with local/regional amendments), effective July 1, 2026.
Code reference
Longmont adopted the 2024 International Codes (including the energy code) along with the Metro Cohort Model Code (MCMC, based on the 2024 IECC with regionally aligned amendments) via Ordinance O-2026-29 (adopted ~June 9, 2026; see Municode Chapter 16.22 for the energy conservation code adoption).Ordinance O-2026-29
Code reference
This aligns with Colorado state requirements for jurisdictions updating codes around this period (must meet or exceed 2021 IECC + electric/solar-ready provisions, with amendments not reducing stringency).
Code reference
Longmont follows its policy of adopting each new IECC version; it is not the Denver Green Code or Energize Denver program.
Code reference
Longmont is in IECC Climate Zone 5B.
Key prescriptive requirements (from the adopted 2024 IECC/MCMC base provisions,
Key prescriptive requirements (from the adopted 2024 IECC/MCMC base provisions, applicable to additions and alterations per the IECC scope for new construction elements in remodels/additions) relevant to remodels/additions in CZ 5B include (specific MCMC amendments may modify details; always verify
Ceiling/attic insulation
R-49 (or equivalent U-factor per Table R402.1.2/R402.1.3).
Adopting ordinance/reference
MCMC/2024 IECC Table R402.1.2 (or R402.1.3 component performance alternative) as adopted in Longmont Municipal Code Chapter 16.22.
Ordinance O-2026-29Source: longmontcolorado.gov

Codes are amended regularly — verify current requirements with **The City of Longmont Building Services (Building Inspection division) issues residential building permits.

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What an ADU or backyard cottage involves

An accessory dwelling unit (ADU), sometimes called a backyard cottage, is a secondary living unit on a lot that already has a primary home. Common forms include detached backyard cottages, garage conversions, and attached additions with a separate entrance and living space. In the Denver metro, feasibility usually turns on zoning and lot standards, utility capacity, access and parking rules, setbacks, height limits, and how the unit will be used (long-term housing, multigenerational living, or flexible guest space). Early planning should confirm what your city or county allows on your specific parcel before design is locked in.

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Design and site factors that drive the project

Strong ADU plans start with the lot, not just the floor plan. Key inputs include buildable area after setbacks, existing structures and easements, grading and drainage, tree and landscape impacts, alley or side-yard access, and where water, sewer, gas, and electric can connect. Interior planning typically covers a compact kitchen, full or three-quarter bath, sleeping space, storage, daylight, and code-required egress. Exterior design should fit the primary home and neighborhood context while meeting height, lot coverage, and exterior material standards set by the local jurisdiction. Detached units also need clear decisions on foundation type, insulation and energy detailing for Colorado’s climate, and durable exterior assemblies that handle freeze-thaw cycles and sun exposure.

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Permits, inspections, and trade coordination

ADU projects generally require building permits and may also need zoning review, utility permits, and separate trade permits. Requirements vary by city and county across the Denver metro, so the submittal package should match the rules for the property address. A municipally licensed Denver general contractor coordinates the overall project and works with licensed specialty trades for work such as electrical, plumbing, and mechanical systems. That coordination covers sequencing, inspection readiness, and keeping architectural, structural, and MEP details aligned as the unit moves from plans to construction.

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Typical build path

Most ADU projects follow a clear sequence: feasibility and code check, measured site review, schematic design, construction documents, permit submittal, procurement, construction, inspections, and closeout. Detached cottages often involve site work, foundation, framing, roofing, windows, insulation, drywall, finishes, and final utility tie-ins. Garage conversions and attached ADUs can reduce some site work but may add structural, fire-separation, moisture, and existing-condition challenges. Conversion projects need careful assessment of foundation, framing, insulation, head height, and whether the existing shell can support a comfortable, code-compliant dwelling.

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What influences cost without a one-size price

ADU cost is project-specific. Major drivers include unit size, detached versus attached or conversion scope, foundation and site work, utility run lengths and capacity upgrades, kitchen and bath layout, finish level, structural complexity, energy detailing, access constraints, and local permit requirements. Soft costs also matter: design and engineering, survey or site documentation when needed, permit fees, and longer lead times for selected materials. A clear scope and early utility/site review usually reduce redesign and change orders later.

FAQ

Common questions

What is the difference between an ADU and a backyard cottage?

They often describe the same idea: a secondary dwelling on a lot with a primary home. “Backyard cottage” usually means a detached unit behind the main house. “ADU” is the broader planning term and can include detached units, attached units, or conversions, depending on local definitions.

Who handles the trade permits and licensing on an ADU project?

A municipally licensed Denver general contractor coordinates the project and the permit path. Licensed specialty trades perform regulated trade work such as electrical, plumbing, and mechanical, and those trades pull the permits required for their scopes where applicable.

Can every Denver-metro lot add an ADU?

No. Eligibility depends on local zoning and lot standards, including use rules, setbacks, height, lot coverage, parking, and utility constraints. The right first step is confirming what the jurisdiction allows for the specific address and lot configuration.

Is a garage conversion simpler than a new detached cottage?

Sometimes, but not always. Conversions may reuse an existing shell and reduce some site work, yet they can require structural upgrades, insulation and moisture improvements, fire separation, ceiling-height solutions, and new utility routing. A detached cottage adds foundation and full exterior scope but can offer a cleaner layout when yard space and setbacks allow.

How long does an ADU project usually take?

Timeline depends on design complexity, utility work, permit review duration, material lead times, weather, and inspection scheduling. Projects with clear feasibility, complete drawings, and fewer existing-condition surprises generally move more predictably than conversions discovered midstream to need major upgrades.

What decisions should homeowners make early?

Confirm intended use, target size, detached versus attached or conversion, access and privacy needs, parking implications, budget range as a planning input, and whether the unit must function independently year-round. Those choices shape site layout, utility design, and the permit strategy.

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How to start planning

Begin with parcel realities: zoning path, buildable envelope, access, and utility capacity. Then define the unit type and must-have program so design and budgeting stay aligned. From there, construction documents and permit coordination can proceed with fewer revisions. If you are comparing options, bring basic lot information, any prior survey or improvement location certificate if available, rough size goals, and notes on how the unit will be used. That preparation helps a general contractor and design team give a clearer read on scope and sequence.

Service Area

Serving Longmont and Boulder County

We serve Longmont, CO and all surrounding Denver metro communities. Our team is based in Denver and operates throughout the Front Range, from Castle Rock to Longmont and everywhere in between. Contact us for a free estimate on your Longmont project.

FAQ

Common questions

Is an ADU allowed on my lot?

It depends on your city’s zoning — lot size, setbacks, and parking rules vary across the metro. We run the zoning check as the first step and tell you what your lot supports before you spend money on design.

Do you handle ADU permits and inspections?

Yes — ADU permitting is municipal, and we manage zoning review, permit application, and every inspection through certificate of occupancy.

Detached, attached, or garage conversion — which is right?

Each has different zoning, cost, and site implications. We design all three and recommend based on your lot, budget, and how you plan to use the unit.

What does an ADU cost?

Getting a real answer costs less than most people expect: an ADU feasibility consultation is $350, and the full ADU feasibility package — zoning check, site analysis, and concept direction — is $950, both listed at fixed prices on our booking page. Construction cost depends on the path: garage and basement conversions reuse existing structure, while a detached backyard cottage is new construction from the foundation up. After feasibility we quote the build itself as a fixed-price proposal, so you know the number before work starts.

How long does an ADU project take?

Three phases run in sequence: design (site measure, concept, construction documents), permitting (zoning and building review, which move on the municipality’s calendar), then construction. Conversions generally move faster than detached new builds because the shell and foundation already exist. We lay out the phase-by-phase schedule in your proposal, and you track progress on our operations platform once work starts.

How small a lot can support an ADU?

Smaller than most people assume — many Front Range single-unit lots that owners write off can carry a detached unit, and each city sets its own minimum lot size, setbacks, and building coverage. Whether your district’s standards leave room for a backyard cottage — or point you toward a garage or basement conversion instead — is exactly what the feasibility check answers for your address.

What is the ADU permit process?

Two municipal reviews run in order: a zoning review confirming the unit fits your district’s standards, then a building permit review of the construction documents, with inspections during the build and a certificate of occupancy at the end. Colorado HB 24-1152 now requires most Front Range municipalities to allow ADUs on single-unit lots, and we confirm how your city implements it as the first step of feasibility.

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