Further reading
An addition is the only remodel that changes the outside of the house, and that single fact is what makes it a different kind of project. Zoning decides whether it can be built at all, geology decides what the foundation costs, and the review takes longer than anything else we do. The construction is the straightforward part.
The Four Kinds of Addition We Build
- Bump-out — 30 to 150 square feet added to an existing room. Often cantilevered off the existing floor framing, which avoids a new foundation entirely. The cheapest square footage in remodeling.
- Room addition — a full single-storey addition on a new foundation. The most common project, and the one where soils and setbacks decide the design.
- Pop-top — a second storey added over an existing house. Common in Denver’s bungalow neighbourhoods. Requires verifying that the existing foundation and walls can carry the new load, and inside a historic district it needs Landmark review.
- Accessory dwelling unit — a separate living unit, detached or converted. Rules vary sharply by jurisdiction and are the first thing to confirm.
Typical planning range
| Type | Planning range |
|---|---|
| Bump-out | $15,000–$45,000 |
| Room addition | $180–$320 per square foot |
| Pop-top / second storey | $250–$400+ per square foot |
Ranges are for planning. We price a project after we have seen it, not before. A full breakdown is in our Colorado home addition cost guide.
Zoning Comes Before Design
Before anything is drawn, four numbers determine what is buildable on your lot: the setbacks from each property line, the maximum lot coverage, the height limit, and the floor area ratio where one applies. They come from the zone district, and they vary not just between counties but between neighbourhoods within a city.
Two local specifics worth knowing. In Denver, if the property is inside one of the city’s 52 historic districts or is individually landmarked, an addition requires a Certificate of Appropriateness from the Landmark Preservation Commission — and the guidelines favour additions that are subordinate and set back from the primary facade. Designing with that in hand is far cheaper than redesigning after a review. In Douglas County and much of the newer south metro, nearly every subdivision is governed by an HOA, and in Highlands Ranch by both a master association and a neighbourhood sub-HOA. Architectural approval there comes before the building permit is worth pulling. We sequence association review first so nothing gets designed twice.
Foundations and Colorado Soil
This is the cost item that surprises people, and it is not optional.
The Colorado Geological Survey has published studies specific to this region — OF-02-08 on soil and bedrock conditions in north-central Douglas County, and SP-42 on heaving bedrock associated with steeply dipping expansive formations. The soils derived from that claystone bedrock hold highly swelling clays that expand and heave when they get wet. The Survey’s guidance is a site-specific geotechnical investigation for each lot or building pad, with at minimum a test boring and swell-consolidation testing at foundation and floor levels.
For an addition, three consequences follow. A soils report belongs in the budget from the start, not as a change order. The foundation design — caissons or drilled piers with a structural floor versus a conventional footing — is determined by that report, and the difference between them is significant money. And drainage around the new foundation is not a finishing touch: downspout extensions, positive grading away from the wall, and keeping irrigation off the foundation are what prevent the movement in the first place, because wetting the clay is the mechanism.
In the foothills the constraint is different. Ground snow load in the Jefferson County mountain areas runs 50 to 80 psf, and any design above 70 psf requires a licensed professional. That means heavier framing, mandatory stamped drawings and a longer schedule than the same addition in the metro.
Wildfire Rules Now Apply to Additions
The Jefferson County Wildfire Resiliency Code took effect on July 1, 2026, and it applies to exterior alterations and additions, not only to new construction. Inside the Wildland-Urban Interface overlay — which covers property above 6,400 feet — that means Class A roofing tested to ASTM E108 or UL 790, ignition-resistant siding, noncombustible deck framing and surfaces, and vents meeting an ember-resistance standard. Properties in the overlay also need a Defensible Space Permit with mitigation inspected by a County-approved forester. The rules sit in Section 39 of the Zoning Resolution and Appendix Z of the building code. Anyone planning an addition in Evergreen, the canyons or the upper foothills should have this priced in from the first sketch.
Permits and How Long They Take
Every addition requires a permit, and additions carry the longest reviews of any residential work. Timelines we plan around:
- Unincorporated Arapahoe County — plan review averages 6 to 8 weeks under $100,000 and 10 to 12 weeks above it, from a complete submittal. Additions require foundation and framing plans stamped by a Colorado registered architect or engineer.
- Unincorporated Adams County — staff review 10 to 20 business days, under the updated International Building Codes effective July 1, 2026.
- Unincorporated Jefferson County — 10 to 20 business days; anything triggering Planning and Zoning review takes longer.
- Parker — the town publishes 1 to 2 weeks for residential additions and remodels, 2 to 4 weeks for new construction.
- Denver — a 180-day target on building permits, though projects under $500,000 with no other agency approval can qualify for a same-day walk-through path.
An incomplete submittal restarts the clock. That is the whole reason we prepare the full package — survey, soils report, stamped structural, energy compliance — before filing rather than filing early and amending.
Tying New to Old
The details that separate an addition that reads as part of the house from one that reads as an addition are mostly decided before the drywall goes on.
The roof connection is the first: matching pitch and tying into the existing framing properly, with correct flashing at every intersection, because a valley between old and new roof planes is where leaks start. Then the foundation joint, where new concrete meets old and where differential settlement is managed rather than wished away. Then the mechanical question nobody asks early enough — whether the existing furnace, panel and water heater can carry the added load. A 100-amp panel in a 1950s house frequently cannot support an addition, and finding that out at the electrical rough is an expensive week. Finally the exterior: matching siding profile, brick colour and window proportion, or making a deliberate contrast rather than an accidental one.
Asbestos and lead are not optional line items
Colorado’s Regulation 8 applies to a house of any age. Inspection by a Colorado-certified asbestos building inspector is required once the work will disturb 32 square feet of material, 50 linear feet on pipe, or the equivalent of a 55-gallon drum. Friable material must be removed by a certified General Abatement Contractor before demolition, and only an accredited laboratory can clear a sample. On pre-1978 homes the federal Renovation, Repair and Painting rule requires certified lead-safe work practices once painted surfaces are disturbed. A bid that omits this is not cheaper — it is incomplete.
Septic Capacity in the Rural Areas
On any property outside the sewer districts, adding a bedroom is a septic question first. In unincorporated Arapahoe County a certificate of sewer availability is required for a new bedroom on an on-site wastewater system. In Douglas County the lot-size rules are firm: a minimum of 21,780 square feet with a public water supply, or a full acre with a private well, with a Level 3 soil survey by a certified soil classifier. Well permits come from the Colorado State Engineer’s Office rather than the county and may carry use restrictions. We pull the septic record before drawing a floor plan, because the design capacity of the existing system can decide the entire scope.
Where We Work
Across the Denver metro and the Front Range: Denver, Jefferson County, Arapahoe County, Douglas County and Adams County. Each has its own permit authority and its own review timeline, and we confirm which one applies to your address before any design work starts.
Home Additions by City
Permit rules, inspections and lead times for home additions change from one jurisdiction to the next. Pick your city:
Counties We Cover
Full service-area coverage by county:
Common Questions
How long does an addition take, start to finish?
Design and engineering typically three to six weeks, permitting anywhere from two weeks to three months depending on jurisdiction, and construction three to six months for a room addition. The honest answer for most Front Range additions is six months to a year overall, and the permitting variance is the biggest unknown.
Do I need a soils report?
On this ground, for anything on a new foundation, plan on it. The Colorado Geological Survey recommends a site-specific investigation per lot, and jurisdictions that enforce it will not accept a foundation design without one. It is a small cost against the cost of a foundation designed for the wrong soil.
Can I add a second storey to my house?
Sometimes. It depends on whether the existing foundation and bearing walls can carry the load, which is a structural engineer’s determination, not a guess. In Denver’s older neighbourhoods pop-tops are common and well understood; inside a historic district they also need Landmark review.
Can I live in the house during construction?
For most room additions yes, since the new structure is largely built before the connecting wall is opened. For a pop-top, generally no. We tell you which before you sign.
What Actually Drives the Cost
Two additions of identical square footage can differ by a third in price, and the difference is almost never the finishes. It is these, roughly in order of impact:
- Foundation type. Drilled piers with a structural floor over expansive soil cost substantially more than a conventional footing on stable ground. The soils report decides this, not the design.
- Roof geometry. A simple gable tying cleanly into the existing roof is straightforward. Multiple intersecting planes, valleys and dormers multiply both labour and leak risk.
- Mechanical capacity. If the existing furnace, panel or water heater cannot carry the addition, those upgrades belong in the addition’s budget.
- Site access. A tight urban lot with no room for equipment, material staging or a dumpster costs more to build on than an open suburban lot.
- Jurisdiction. Longer reviews and stamped-drawing requirements are real cost, not paperwork.
- Wet rooms. An addition containing a bathroom or kitchen carries plumbing, venting and waterproofing that a bedroom addition does not.
Should You Add On or Finish the Basement?
Worth asking honestly before spending anything. A basement finish typically runs a fraction of the per-square-foot cost of an addition because the structure, roof and foundation already exist. What it cannot give you is natural light on more than one side, standard ceiling height in an older house, or an at-grade connection to the yard. Egress windows are required for any bedroom below grade and are a real cost item. If the need is a family room, an office or a guest suite, the basement is often the better value. If the need is a primary suite, a bigger kitchen or a room that connects to the outdoors, only an addition delivers it. We will tell you which one your goal actually calls for, including when the answer costs us the larger job.
Recent Work
Real photos from a completed garage conversion / adu project in Littleton, CO.
Related Services
Kitchen remodeling · Bathroom remodeling · Framing · Roofing · Colorado remodeling price guide
For how an addition gets paid for, see our payment options page.