Detached, Attached, or Interior ADU? Choosing the Right Type for Colorado

At a Glance

A quick scan if you don’t have time for the full guide:

ADUs come in three types: detached (standalone backyard), attached (shares a wall with main house), and interior conversion (basement, garage, attic, or JADU built into existing structure).
All-in cost ranges: detached $150K to $400K typical (modular at the low end; stick-built on larger or tougher lots can stretch to $450K), attached $125K to $300K, interior conversion realistically $100K to $200K once code upgrades land.
Detached units rent for 15 to 25% more than attached and carry the biggest resale premium.
Modular construction works for detached only. Attached and interior are stick-built by definition.
Garage conversion economics vary sharply by city. Fort Collins removed parking mandates and is the most permissive market. Denver still requires parking compensation and adds significant cost.
Basement conversion is Colorado’s most common interior path because most Colorado homes have full basements.

Why ADU Type Is the First Real Decision

Homeowners weighing ADU options often anchor too narrowly. Attached looks cheaper than detached. Lower foundation cost, shared wall, some shared utilities. The sticker price reads lower, the build pencils look lower, and the choice seems made.

But anchoring on one number alone, usually the upfront price, leaves a lot on the table. Detached costs more upfront. It also rents for 15 to 25% more per month, carries a larger resale premium, offers full privacy for both you and whoever uses the unit, and holds its value across more use-case shifts over the decades you’ll own the property. Over a five-year horizon, detached usually pencils out better even after the higher build cost.

The real question isn’t “which is cheaper to build.” It’s “which actually delivers what I want over the lifetime I’ll own this property.” This guide walks the three ADU types (detached, attached, and interior conversion) with honest cost ranges, real rental and resale implications, and the Colorado-specific factors that shift the math.

Whether you’re building in Boulder, Denver, Fort Collins, or one of Colorado’s mountain communities, the same three choices apply. The right answer just shifts based on your specific lot, goal, and budget.

The Three ADU Types

Three categories. Each has its own cost profile, build method options, code complexity, and best-fit use case.

Detached. A standalone structure built in your backyard with its own foundation, roof, and utility connections. Most flexible for rentals because tenants get full privacy. Highest upfront cost because you’re building a complete second structure. Build methods available: both modular and stick-built. Modular detached is built in a factory and craned onto your foundation. Stick-built detached is framed entirely on your lot. Modular runs 3 to 6 months at a fixed price. Stick-built runs 12 to 18 months and pricing typically is not fixed.

Attached. Shares at least one wall with your main house. Can be a bump-out addition, an upper-level expansion, or a tied-in garage modification. Cheaper than detached because you reuse a wall and often share utilities. Build method: stick-built only, because attached units have to integrate physically with your existing structure during construction. Less privacy due to the shared boundary. Stick-built timeline: 9 to 15 months.

Interior conversion. Built inside an existing structure. Build method: stick-built only, since by definition you’re working within existing footprint that can’t be factory-built. Covers four subtypes: basement conversion (most common in Colorado homes with full basements), garage conversion (the most code-complex), attic conversion (less common, mostly older homes), and JADU (Junior ADU, a California-coined category for units under 500 sq ft carved from existing bedroom space). Timeline runs 5 to 10 months depending on subtype and structural work required. JADU is industry shorthand and not a Colorado regulatory category, but the term appears often enough in builder conversations that it’s worth recognizing.

Detached ADUs: The Income and Privacy Default

Realistic all-in cost: $150K to $400K typical, with stick-built builds on larger or more challenging lots reaching $450K. Modular detached runs $150K to $350K with a faster, fixed-price build. Stick-built detached runs $200K to $450K with more design flexibility but variable pricing.

Why detached wins for rental income. A 500 to 700 sq ft Front Range detached ADU pulls $1,650 to $1,900 per month long-term, 15 to 25% higher than the equivalent attached unit. Tenants pay the premium because they get full separation from your living space, separate entrance, separate utilities, and a real sense of being in their own home.

Why detached wins for resale. Boulder homes with detached ADUs sell for 20 to 35% more than comparable homes without one. The premium is consistent across the Front Range. If you build a detached ADU and never rent it, the resale bump alone often justifies the project.

The trade-off. Higher upfront cost, longer build timeline if stick-built (12 to 18 months versus 3 to 6 for modular), more lot footprint consumed. Detached requires roughly 1,500 to 2,000 sq ft of usable lot space when you factor setbacks. Our how-to-build guide for Colorado walks the full detached construction process.

Realistic all-in cost: $125K to $300K depending on whether you’re doing a bump-out, an upper-level addition, or a tied-in garage modification. Lower than detached because you share at least one wall, often share utility runs, and skip the second-foundation cost.

Here’s the framing most articles get wrong: attached isn’t the obvious family-housing default. Most families actually prefer detached even on the same lot, because detached gives separation alongside proximity. A parent or adult child living 30 feet from your kitchen window in their own structure has a meaningfully different daily experience than one living through your shared wall. The privacy compounds across years.

Where attached actually wins is in specific cases, not as a general default:

Kids’ room extensions where privacy isn’t paramount and proximity is actually welcomed.
Aging parents with safety concerns where immediate access through a shared wall matters more than separation.
Lots too small for detached (under roughly 6,000 sq ft after setbacks).
Budgets that genuinely cannot stretch to detached, with the family member or tenant relationship comfortable with the shared boundary.
Historic district restrictions that complicate new exterior structures.
Properties where the addition serves the primary home as much as the ADU (a master suite that doubles as a future rental unit, for example).

Where attached struggles is the rental market. Tenants pay 15 to 25% less for attached units because the shared wall reduces sound separation and the psychological sense of being in their own space. Resale can also be trickier; some buyers hesitate about living adjacent to a tenant unit. If your primary goal is rental income, attached is rarely the best play.

The Colorado wrinkle. Some Colorado jurisdictions classify large attached additions as primary-home expansions rather than ADUs, which can change permit requirements and tax assessments. Confirm classification with your local planning department before designing.

Interior Conversions: The Existing-Space Play

This is where the math gets interesting, and where the marketing tends to get misleading.

The headline cost ($40K to $150K) is rarely the real cost. That shell-only number is genuine for the basic conversion work: drywall, finishes, basic plumbing extension, light electrical. But almost no interior conversion stays at the shell-only stage once code review begins. Realistic all-in cost for a code-compliant interior conversion lands at $100K to $200K.

Basement conversion is Colorado’s most common interior path. Most Colorado homes have full basements that are already partially finished. The conversion adds: a separate entrance (often a walk-out staircase or door modification, $5K to $15K), a code-compliant kitchen ($15K to $30K), a separate electrical sub-panel ($5K to $8K), upgraded egress windows for bedrooms ($3K to $8K each), and HVAC zoning ($4K to $10K). Realistic basement conversion: $90K to $160K all-in.

Garage conversion is the most code-complex subtype. The trap is ceiling height: most Colorado garages have 8-foot ceilings, but ADU code typically requires 7-foot-6-inch minimum from finished floor to lowest ceiling point. Drop beams, HVAC ducts, and sloped ceilings often fail this requirement. Insulation is another constraint; garage walls are rarely insulated to livable standards. Separate electrical service runs $5K to $10K. Then there’s parking compensation: if your garage conversion eliminates required off-street parking, you may need to build new dedicated parking elsewhere on your lot, which can add $10K to $30K.

Garage conversion economics vary sharply by Colorado city. Fort Collins removed parking mandates entirely after HB24-1152, which directly enables garage conversions because owners no longer have to compensate the lost garage parking. The same conversion that lands at $80K in Fort Collins can hit $150K in Denver once parking compensation construction is added. Boulder’s tight setbacks add their own constraint. If you’re considering a garage conversion, check your specific city’s parking and setback rules before pricing the project. Our Denver, Springs, and Fort Collins comparison breaks down what each city allows.

Attic conversion is less common in Colorado but real, mostly in older homes with usable attic volume. Ceiling height, structural reinforcement, and egress are the constraints. Realistic cost: $80K to $140K for a small unit.

JADU. California-coined category for an interior unit under 500 sq ft carved from existing bedroom space, sharing some utilities with the primary home. Not a Colorado regulatory category, so JADU treatment varies by jurisdiction. Most Colorado cities treat it as a small interior conversion subject to the same code requirements as any other.

When interior conversion makes sense. Tight lots where detached won’t fit. Homeowners with already-finished or partially-finished basements. Projects where preserving exterior aesthetics matters (historic districts especially). Family-use cases where proximity is a feature, not a trade-off.

When it doesn’t. When the realistic all-in number creeps past $180K, detached often becomes more attractive on a dollar-per-sq-ft basis. When your goal is maximum rental yield, interior usually underperforms detached.

Side-by-Side: How the Three Types Compare

Type	Build method	All-in cost	Build time	Rental premium vs detached modular	Privacy	Best for
Detached	Modular	$150K to $350K	3 to 6 months	Baseline (highest rent)	Highest	Income, speed, fixed-price predictability
Detached	Stick-built	$200K to $450K	12 to 18 months	Same baseline	Highest	Custom design, unique site needs
Attached	Stick-built only	$125K to $300K	9 to 15 months	15 to 25% lower	Medium	Multigenerational family, smaller lots
Interior conversion	Stick-built only	$100K to $200K	5 to 10 months	20 to 30% lower	Lower (depends on subtype)	Tight lots, family use, existing-space economics

The table assumes a typical Front Range lot with standard utilities, no significant slope, and a mid-range finish level. Mountain lots, historic district lots, and lots with utility distance issues shift these numbers upward across all types and methods.

Colorado Lot Factors That Change Everything

The type that’s right for your specific lot depends on factors that often outweigh the type itself.

Lot size and setbacks. Detached ADUs need roughly 1,500 to 2,000 sq ft of usable backyard space after setbacks. Smaller lots (under 6,000 sq ft) often can only support attached or interior conversion. Boulder’s older neighborhoods like Whittier and Mapleton Hill frequently fall into this category.

Slope and soil. Steep lots double excavation costs for detached. Expansive clay soils across the Front Range require engineered foundations that add $10K to $20K to detached projects but don’t affect interior conversions at all. Mountain bedrock excavation can add $20K to $50K to any new foundation work.

Utility distance. Detached projects on sprawl or mountain lots with utilities far from the building pad can add $20K to $50K in trenching. Attached and interior conversions tap into existing utility runs and sidestep most of this cost.

HOA and zoning. HB24-1152 strips HOA bans, but local design standards still apply. Some jurisdictions treat attached additions and detached units differently for setback and height purposes. Always confirm with your local planning department. Our HB24-1152 explainer walks the full legal context.

For deeper coverage of how your specific lot affects the build, our how-to-build guide for Colorado has a full lot-personality treatment.

How to Pick the Right Type for Your Lot

Walk through three questions in order. Each one narrows the options, and the answer to one determines what matters in the next.

Step 1: What does your lot allow?

This is the gate before anything else. Lot space dictates which types are even on the table, in this priority order.

Detached needs roughly 1,500 to 2,000 sq ft of usable backyard space after setbacks. Front Range suburban lots typically have this. Small Boulder historic lots often don’t.

Attached needs less raw space, but requires room to bump out off your existing structure or convert an integrated garage. Lot geometry matters more than total size.

Interior conversion needs no new footprint. Uses your existing structure. If your basement, attic, or garage can support legal conversion, this works on any lot.

None of the above is a real outcome. If your lot can’t support any ADU type, the path forward is either expanding your primary home (which is not an ADU and carries different permit and financing rules) or looking at a different property entirely. Lot expansion (buying adjacent land, splitting an oversized neighboring parcel) is technically an option but rarely realistic outside specific circumstances.

A property check confirms which types your specific lot supports before you spend time on the next step.

Step 2: Match what your lot allows to your goal

This is where build method (modular vs stick-built) matters. The right answer depends on whether your goal aligns with what the lot lets you build.

Lot allows detached, goal is rental income: modular detached is usually the cleanest path. Fixed price, 3 to 6 month timeline, highest rental premium. Stick-built detached is the alternative if you need custom design beyond what modular floor plans offer.

Lot allows detached, but a specific case favors attached: situations like an extension for kids’ rooms, an aging parent needing immediate proximity, or budget pressure that makes attached the realistic ceiling. Attached saves money and footprint here, with the trade-off of reduced privacy and rental premium.

Lot ideally needs detached but doesn’t allow it: check whether lot expansion is realistic before you settle. If not (and usually it isn’t), revert to attached as the next-best option. Attached gives you most of the use case at a lower cost. The trade-off is mainly privacy and rental premium.

Lot only allows interior, but you want tenant separation: interior conversion may not deliver what you want. The honest options are accepting the reduced separation, choosing a basement-with-walk-out-entrance configuration for as much privacy as the type allows, or stepping back to consider whether a different property serves your long-term goal.

Lot allows all three, goal is family use: most families pick detached here for the privacy-with-proximity combo. Attached makes sense when immediate access matters (aging parent safety, young kids’ bedroom expansion) or when budget forces the choice. Interior conversion when exterior changes aren’t desirable (historic districts, HOA aesthetic limits).

Step 3: Match your realistic budget to your goal

If steps 1 and 2 land you on a type and method you can afford, you’re done. If they don’t, you have options as a human, not just as a buyer.

Detached desired but only attached budget allows: options include downgrading to attached now, saving up for detached over the next 1 to 2 years, or financing the gap with a construction loan or HELOC. Construction loans against an ADU typically work because the structure is real estate, not personal property.

Detached budget covers modular but not stick-built: good fortune. Modular is usually cheaper and faster for the same square footage. Pick modular. The “downgrade” actually saves you money and shortens your timeline.

Lower budget, interior conversion path: consider basement first if you have one (cheapest interior at $90K to $160K all-in). Garage conversion is the most variable on cost because of code traps. Attic and JADU are smaller-scope plays that work when even basement is too much.

No type fits your budget honestly: the option no one talks about is delaying. The ADU is still a real choice in 18 months. Saving $20K to $50K in that window can shift you from “barely affords interior” to “comfortably affords detached,” which often pencils out better even with the wait.

If all three steps land cleanly, you’ve picked your type with eyes open. If they don’t, the bottleneck (lot, goal, or budget) is the next thing to address before you commit.

Where Olerra Fits In

We build detached modular ADUs across Colorado, exclusively. Not because attached and interior conversions are bad choices. They’re the right choice for many homeowners. We just don’t build them, because our factory-build model only works for detached units that get craned onto a foundation.

If detached is the right answer for your lot, our process page walks the full five-step sequence: property check, fixed-price quote, permits, parallel factory build and site prep, single-day install. Three to six months from contract signing to keys.

If attached or interior is the right answer, we’ll tell you so during the property check. There’s no value in selling you a detached unit on a lot or budget that doesn’t support one.

→ Get your free property check and quote.

FAQ

Can I switch ADU types mid-project?

Not easily. Switching from attached to detached (or vice versa) requires re-permitting and often re-engineering. Switch the type during the design phase, not after.

Do all three types count under HB24-1152?

Yes. HB24-1152 requires Subject Jurisdictions to allow at least one ADU on every single-family lot, regardless of type. Local size, setback, and design standards still vary by type.

Will an interior conversion appraise the same as a detached?

Usually less. Detached ADUs typically appraise higher per square foot because they’re recognized as fully separate dwelling units. Interior conversions often get partially credited as additional finished space rather than a full ADU. Confirm with a local appraiser.

Can I rent any of the three types short-term (Airbnb)?

Depends on your city. Boulder restricts most new ADU short-term rentals across all types. Denver requires owner-occupancy for STR. Colorado Springs prohibits STR on properties with ADUs as of June 30, 2025 (grandfathered properties may continue). Fort Collins allows STR without owner-occupancy.

Is a JADU a Colorado regulatory category?

No. JADU is California-coined industry shorthand for an interior unit under 500 sq ft carved from existing bedroom space. Colorado doesn’t formally recognize the JADU category in HB24-1152. Most Colorado cities treat JADUs as small interior conversions.

What if my lot can’t fit a detached ADU?

Attached or interior conversion. Both are legally permitted on lots too small for detached. Interior conversion is often the cleanest path because it doesn’t require any new footprint.

Does Olerra build attached or interior conversion ADUs?

No. We build detached modular only. If your situation calls for attached or interior, we’ll say so and you’ll need a different builder.

Will my property taxes go up the same amount regardless of type?

Roughly, yes. Colorado assesses ADUs as additional square footage. The assessment increase tracks finished sq ft added, so a 500 sq ft detached and a 500 sq ft basement conversion typically generate similar property tax impact.

Find Your Fit

You’ve seen the trade-offs. The type that’s right for your Colorado lot depends on your goal, your budget, and what your specific lot supports.

Detached for income and privacy. Attached for family. Interior for tight lots and existing-space economics.

No spec house. No long sales call. No mystery pricing.