If you've searched for an ADU builder in Denver, Aurora, Lakewood, or anywhere on the Front Range, you've probably noticed the advice online is generic — written for "California ADUs" or "national trends," not for the specific patchwork of city halls you actually have to deal with. Colorado changed the zoning picture in 2024, but zoning is only step one. Here's what building an accessory dwelling unit in the Denver metro actually involves, city by city.
What Counts as an ADU
An accessory dwelling unit is a second, independent living space on a lot that already has a primary single-family home — a backyard cottage, a unit built over or attached to a garage, an addition connected to the main house, or a conversion of existing space (basement, garage) into a legal second unit. It has its own entrance, its own kitchen, and its own bathroom, and it can be rented or used by family separately from the main house.
As of 2024, Colorado has a statewide law on this: House Bill 24-1152, signed May 13, 2024. It requires "subject jurisdictions" — municipalities of a minimum size inside metropolitan planning areas, which covers essentially the whole Denver metro — to allow at least one ADU per single-unit detached home through an administrative approval process, and it bars those jurisdictions from enacting rules that restrict ADU construction or conversion beyond that. In plain terms: your city can no longer tell you ADUs simply aren't allowed on your lot. That's a real shift from a few years ago, when large swaths of the metro had zoning that blocked them outright.
What the state law does not do is standardize how each city implements that requirement, who reviews your plans, or who is licensed to build it. That part is still entirely local — and it's the part that trips people up.
Zoning Check First — Because It's Different in Every City
HB24-1152 sets the floor (cities must allow an ADU), but the ceiling — lot coverage limits, height and bulk-plane rules, parking requirements, setbacks, and short-term-rental restrictions — is still set city by city. So is the permitting authority, and so is contractor licensing. Colorado has no statewide general contractor license; GC licensing is issued municipally, city by city or county by county. A contractor licensed to pull permits in Denver isn't automatically licensed in Aurora or Lakewood. That's exactly why the first real step on any ADU project is a zoning and permitting check specific to your address — not a generic "ADUs are legal now" assumption.
Here's who actually issues residential building permits across the metro, and who you're dealing with once you're past the zoning question:
- Denver — Community Planning and Development issues permits; contractor licensing is municipal, so a Denver-licensed contractor is what pulls Denver permits.
- Aurora — the Building Division's Permit Center issues permits, with applications routed through the city's online portal; the Building Division also routes contractors through Aurora's own contractor licensing and lookup process.
- Lakewood — permits run through Public Works, and contractors need to be registered with the city and hold a Lakewood contractor account before permits are released.
- Littleton — the Community Development Department's Building Division and Permit Center requires a license or registration certificate to pull permits within city limits.
- Centennial — the Building Division handles permits, inspections, and contractor license approval through the city's online Self-Service Portal.
- Arvada — Building Services issues permits and explicitly notes that unpermitted work can jeopardize a contractor's licensing standing as well as your property's resale value.
- Westminster — the Building Division requires general contractors (and mechanical/plumbing/electrical subs) to be registered with the city before permit release.
- Highlands Ranch — unincorporated, so permits go through Douglas County's Building Division, and the county's Contractor Registration Program requires registration before you can apply.
Eight cities, eight separate front doors. A builder who is commercially licensed and insured in your specific jurisdiction — not just "licensed somewhere in Colorado" — is a real qualifying question to ask before you sign anything.
The Permit and Inspection Flow (Same Bones, Different Paper)
Once zoning is confirmed, the sequence looks broadly similar across every city in the metro, even though the specific portal, forms, and review timelines differ:
- Zoning and feasibility review. Confirm your zone district, lot coverage, setbacks, height limits, and utility locations against your specific address before spending money on design.
- Design and engineering. Architectural drawings and structural engineering sized to your city's submission requirements — some jurisdictions review zoning, building, and utilities as separate tracks; others bundle them.
- Permit submission and plan review. Your city's building department (see the list above) reviews the drawings. Complete, clean submissions move through review meaningfully faster than incomplete ones — that's true in every jurisdiction we work in.
- Construction with staged inspections. Foundation, framing, rough-in (electrical/plumbing/mechanical), insulation, and final — each triggers a scheduled city inspection before the next phase can proceed.
- Final inspection and certificate of occupancy. Only after this is issued is the unit legal to occupy — or to rent.
The step that most often stalls a project isn't construction — it's incomplete or inconsistent paperwork bouncing back and forth between applicant and reviewer. A builder who submits complete, jurisdiction-correct packages the first time removes most of that friction.
Detached, Attached, or Garage Conversion — Which Fits Your Lot
Under HB24-1152's administrative-approval framework, the three common ADU types are treated as variations on the same basic use, but they carry different practical tradeoffs on your specific lot:
- Detached (backyard cottage). A freestanding structure, usually in the rear yard. Gives you the most design flexibility and the cleanest separation from the main house, but uses the most new foundation and utility work.
- Attached. Built as an addition connected to the primary home, sharing at least one wall. Can be more efficient to build since it ties into existing structure and utilities, but changes the footprint and massing of the main house and is more exposed to that jurisdiction's bulk-plane and setback rules on additions.
- Garage conversion or above-garage unit. Repurposes an existing structure instead of building new. Often the fastest path to a legal second unit if the existing garage's slab, framing, and foundation can support living space — but older garages frequently can't without structural work, which is something a feasibility review should catch before you commit to the approach.
Which one makes sense depends on your lot size, your existing structures, and your city's specific size and setback caps — which is exactly why the zoning check comes before the design decision, not after.
How Design-Build Compresses the Timeline
The traditional path — hire a designer, get drawings, then shop those drawings to contractors for bids — means zoning questions, engineering problems, and permitting requirements often surface after you've already paid for a design that doesn't fit what your city will actually approve. Design-build collapses that into one accountable team: the same firm handles the zoning and feasibility check, the design, the engineering, the permit submission, and the construction. Problems that would otherwise surface mid-permit-review — a bulk-plane conflict, a setback issue, a utility capacity question — get caught during feasibility instead, when a design correction is cheap, not after you've already submitted for permit review.
It also means one point of contact carries your project through every city department it touches, instead of you coordinating between a separate architect, engineer, and contractor across a process that already has enough separate authorities to deal with (see the list above).
Start With a Zoning and Feasibility Check
Every ADU project on the Front Range starts the same way: confirming what your specific lot, in your specific city, actually allows — before any design work begins. See how we handle ADU and backyard cottage projects, or get a free estimate and we'll walk you through what your address qualifies for.
Related Reading & Recent Work
- ADU & Backyard Cottage Design-Build in Denver — our full design-through-permit ADU service
- Marshello ADU — currently building in Denver — a real ADU moving through this exact permit flow right now
- Why Denver Homeowners Are Investing in ADUs in 2026
- Ready to check your lot? Start with a zoning and feasibility review