Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
Before
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Before
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Before
AfterTrusted Home Additions & ADU for Mountain View Homeowners
Home Additions & ADU in Mountain View run $45,000-$130,000 for room additions and $90,000-$220,000 for detached ADUs, with most projects quoted in writing before any permit application begins. Rowan Construction Group has delivered 4,000-plus projects since 2007, every one with a flat written price and signed scope under The Haven Standard. In Mountain View’s competitive housing market, where median home values exceed $2 million, adding square footage through a permitted addition or accessory dwelling unit often costs less than trading up to a larger home.
We’re led by Grant Rowan, and our crews are background-checked, uniformed, and arrive with a documented photo record as standard on every visit. A live person answers our phones 24/7. Call (669) 215-4852 to schedule your free estimate.
What Our Home Additions & ADU Service Includes
ADU Design & Permitting
An accessory dwelling unit starts with a feasibility study and a set of permit-ready drawings, not a handshake. In Mountain View, ADU permits must comply with both the city’s local ordinance and California’s statewide ADU law, which limits parking requirements and setback rules that once blocked projects. We prepare the structural, electrical, and plumbing plans, coordinate with Mountain View’s Building Division for plan check, and do not begin construction until permits are issued and the written price is signed. Under Clause 1 of The Haven Standard, the number you see is the number you pay, with change orders signed before any additional work proceeds.
Garage Conversion ADU
Converting an existing garage to an ADU is often the fastest path to rental income or multigenerational housing in Mountain View, where lot sizes in neighborhoods like Old Mountain View and Monta Loma rarely accommodate new detached structures. The project involves structural assessment of the existing slab, upgrading or replacing the roof system, insulating to Title 24 energy standards, and running new electrical and plumbing from the main service. We typically see garage conversions in Mountain View range from $30,000-$80,000 depending on whether the foundation and roof structure can be retained. Our written quote specifies which elements are existing and which are new, so there is no ambiguity about what the price covers.
New Detached ADU
A new detached ADU is a ground-up structure on a separate foundation, governed by Mountain View’s zoning for rear-yard coverage and height limits. These projects require full site prep, utility trenching, and often upgraded electrical service to the main residence to handle the additional load. In the San Antonio and Rex Manor neighborhoods, we’ve built detached ADUs - Sunnyvale style above 800 square feet with full kitchens and stacked laundry, while in the Jackson Park area we’ve completed smaller studio units under 500 square feet. The $90,000-$220,000 range reflects foundation type, roofing material, and whether the main panel requires upgrade.
Junior ADU & Interior Conversion
A Junior ADU, or JADU, is created within the walls of an existing single-family home, with a separate entrance and efficiency kitchen, sharing bathroom facilities with the main residence. Mountain View allows JADUs up to 500 square feet under state law, and they do not require dedicated parking. Interior conversions also include adding bedrooms, expanding kitchens into underutilized spaces, or creating home offices from formal living rooms that no longer fit how Mountain View families actually live. We assess load-bearing walls, HVAC capacity, and egress requirements before quoting, so the written price accounts for structural engineering if a wall must be opened or replaced with a beam.
ADU Site Prep & Utilities
Site preparation for an ADU in Mountain View often involves more than grading. The clay-heavy soils in parts of the city, particularly near Permanente Creek, may require engineered fill or moisture barriers beneath the slab. Utility work includes trenching for sewer lateral connection, upsizing water service if the existing line is galvanized or undersized, and running electrical conduit from the main panel or installing a separate meter. We coordinate with PG&E and the Mountain View Public Works Department for service upgrades, and our quote specifies whether utility work is included or billed separately if scope changes after city review.
Room Additions
A room addition extends the footprint of an existing home, typically adding a bedroom, family room, or expanded kitchen. In Mountain View’s 1950s and 1960s ranch neighborhoods, these projects often involve matching the existing roofline and foundation type, whether that’s a raised perimeter foundation or slab-on-grade. We assess the existing structure for compatibility, design the addition to avoid step-downs between old and new floor elevations, and specify in the written quote whether the roof is extended or replaced entirely. Matching the weep screed, stucco texture, and window style to the existing home is standard, not an upgrade.
Second Story Additions
Adding a second story is the most structurally intensive addition type, requiring verification that the existing foundation and first-floor walls can carry the new load. In Mountain View, many homes built before 1970 have perimeter foundations and wall framing that must be reinforced or replaced to meet current seismic codes. We bring in a structural engineer early, include their report in the permit set, and quote the foundation work separately from the framed second story so you understand where the money goes. The $45,000-$130,000 range for additions extends significantly for second-story work; we provide line-item pricing so you can phase the project if needed.
Sunrooms & Enclosed Patios
Sunrooms and enclosed patios occupy a middle ground between outdoor living and conditioned space, and their classification affects permit requirements and energy code compliance. A three-season room with no heating or cooling requires less engineering than a four-season space tied into the home’s HVAC system. In Mountain View’s mild climate, many homeowners opt for high-performance glazing and minimal heating, but we specify the thermal performance of every window and door in the quote, including brand and U-factor. Whether you choose ECO Window Systems or another manufacturer, the specification is written, not assumed.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Brands We Service for Home Additions & ADU
We’ve installed CWS windows in dozens of Mountain View additions, particularly their vinyl and aluminum-clad lines specified for Title 24 compliance. The CWS products we quote carry documented NFRC ratings, and we keep installation details on file for warranty registration so the homeowner doesn’t have to track paperwork. For projects requiring impact-rated or high-wind glazing, ES Windows has been our specification for coastal-exposed Mountain View properties near the bay, with laminated glass packages that meet the structural calculations without overbuilding the frame.
Mr-Glass specialty glazing systems appear in our sunroom and enclosed patio projects where homeowners want sliding or folding glass walls that open the addition completely to the yard. We’ve completed Mr-Glass installations in the Waverly Park and Springer Ranch areas, and our crews are trained on their specific hardware adjustment procedures. Whether you have CWS, ES Windows, Mr-Glass, or any other make, we can source, install, and warranty the product under our 365-Day Done Right Promise.
Before
AfterSigns You Need Home Additions & ADU Right Now
- Your household has outgrown the bedrooms you have. In Mountain View, where families often buy modest homes and stay as children arrive, the pressure point is real. We’ve converted dining rooms to bedrooms, added second stories over existing garages, and built detached ADUs for returning adult children. The cost of moving to a larger home in the same school district typically exceeds the cost of adding square footage by a significant margin.
- You have aging parents who need proximity but independence. Multigenerational living is increasingly common in Mountain View, and an ADU or JADU preserves privacy while keeping family on the same property. The alternative, assisted living in Santa Clara County, runs $6,000-$10,000 monthly. A garage conversion ADU amortizes against that expense in under five years.
- Your garage or basement is underutilized storage. Mountain View homes average $1,200-$1,800 per square foot in value. A 400-square-foot garage conversion adds usable living space at roughly $75-$200 per square foot constructed, a fraction of acquisition cost. If the structure is structurally sound and the slab is adequate, the conversion is typically the highest-return addition type.
- Your home office competes with household noise. The shift to remote work has made dedicated workspace a functional requirement, not a luxury. We’ve added standalone office additions in the Blossom Valley and Sylvan Park neighborhoods, and converted underused formal living rooms to enclosed offices with sound-rated doors. The written quote specifies STC ratings and door hardware so you know what separation you’re buying.
- Your lot allows ADU construction but the ordinance window may shift. California’s ADU laws have expanded access repeatedly, but local implementation varies. Mountain View’s current ordinance allows detached ADUs up to 1,200 square feet on most single-family lots, with reduced setbacks. Waiting risks zoning changes, material cost inflation, or permit backlogs. We assess your lot’s specific eligibility during the free estimate and quote the permit timeline based on current Mountain View Building Division processing times.
Our Home Additions & ADU Process - Step by Step
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Site visit and written estimate. We arrive at your Mountain View property, measure the existing structure, photograph conditions, and discuss your goals. Within 48 hours, you receive a flat written price with line-item scope, material specifications, and timeline. This is Clause 1 of The Haven Standard: the price is written before anyone starts. No exceptions.
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Design development and permit submission. Once you approve the estimate, we prepare or refine construction drawings, coordinate structural engineering if required, and submit to Mountain View’s Building Division. We track plan check comments and revise without additional fees unless the scope changes at your request. Every revision is documented and re-signed.
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Pre-construction walkthrough and scheduling. Before breaking ground, we walk the site with you, mark utility locations, establish access routes for material delivery, and confirm the photo documentation process. You receive a project schedule with milestone dates and a single point of contact. Our crews are background-checked and uniformed; you know who is arriving and when.
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Construction with documented progress. Every visit includes a dated photo record uploaded to your project file, showing what was found, what was done, and what comes next. Change orders, if needed, are written, priced, and signed before the additional work proceeds. This administrative discipline, inherited from Haven’s acquisition of Rowan, is why our projects finish within a few percent of the agreed price.
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Final inspection and 365-Day Done Right Promise. We coordinate all city inspections, correct any punch-list items immediately, and deliver your final documentation: permit sign-off, warranty terms, and photo record. If anything is not done right, we make it right under The Haven Standard, backed by a terms page you can reference, not an asterisk.
How Much Does Home Additions & ADU Cost in Mountain View?
A typical room addition in Mountain View runs $45,000-$130,000 depending on whether the project is a ground-level bedroom, a kitchen expansion, or a second story requiring structural reinforcement. ADU construction ranges from $90,000-$220,000 for detached units, with garage conversions at the lower end ($30,000-$80,000) and new construction with utility upgrades at the upper end. Junior ADUs and interior conversions typically fall below the full addition range but require careful assessment of existing HVAC and electrical capacity.
| Project Type | Typical Range in Mountain View | Primary Cost Drivers |
|---|---|---|
| Room Addition (ground level) | $45,000-$85,000 | Foundation type, roofing tie-in, finish level |
| Room Addition (second story) | $80,000-$130,000 | Structural reinforcement, stair access, seismic upgrade |
| Garage Conversion ADU | $30,000-$80,000 | Slab condition, roof replacement, utility routing |
| New Detached ADU | $90,000-$220,000 | Size, foundation, utility trenching, electrical service upgrade |
| Junior ADU / Interior Conversion | $25,000-$75,000 | Kitchenette, separate entrance, shared vs. new bathroom |
| Sunroom / Enclosed Patio | $15,000-$50,000 | Glazing specification, HVAC tie-in, foundation |
Several factors move prices within these ranges. Foundation work is the largest variable: a slab that cracks or settles requires engineered repair before framing begins. Roofing complexity matters; tying into an existing gable roof with matching pitch is simpler than intersecting rooflines. Finish selections affect the final number, but our written quote specifies allowance ranges for flooring, fixtures, and cabinets so you can calculate upgrades without guessing. We source across fifty-plus vetted brands, from KraftMaid and Wellborn cabinets to Kohler and Toto fixtures, which keeps our quotes honest rather than inflated with single-source markups.
To avoid overpaying, get competing quotes in writing and bring them to us for a free second opinion. We review line by line at no charge, flagging scope gaps or inflated allowances. Our estimate includes permits, inspections, and the documented photo record. Nothing is held back as an “optional” add-on that appears mid-project.
Home Additions & ADU Near Mountain View - Our Service Area
Rowan Construction Group builds throughout the Peninsula and South Bay, with crews regularly in Mountain View, Los Altos, Sunnyvale, Palo Alto, Los Altos Hills, Stanford, Cupertino, Santa Clara, Menlo Park, Newark, Atherton, and Saratoga. Most Mountain View properties receive an initial site visit within two business days of calling, and our project schedules account for permit processing times specific to each city’s Building Division. Whether you’re in the Birdland neighborhood near Stevens Creek or the San Antonio area closer to El Camino Real, we know the local soil conditions, setback requirements, and utility constraints that affect addition and ADU feasibility.
Serving Mountain View, CA - Our Local Coverage Area
We’re based in the Mountain View area and also offer our Home Additions & ADU in Los Altos; we know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
Frequently Asked Questions - Home Additions & ADU in Mountain View
A home addition extends your existing residence, sharing walls, roof, and utilities, while an ADU (accessory dwelling unit) is a separate living space with its own kitchen, bathroom, and entrance. In Mountain View, ADUs can be attached, detached, or converted from existing space like a garage, and they are regulated by both state law and local ordinance, similar to Home Additions & ADU - Stanford requirements. Call (669) 215-4852 and we’ll assess which option your lot and goals support.
Permit processing in Mountain View typically takes 8-16 weeks for additions and ADUs, depending on plan check complexity and revision cycles. Construction ranges from 3-4 months for a garage conversion to 8-12 months for a new detached ADU with utility upgrades. Our written quote includes a timeline with permit and construction phases separated, so you understand where time is spent before work begins.
ADU construction in Mountain View ranges from $90,000-$220,000 for detached units, $30,000-$80,000 for garage conversions, and $25,000-$75,000 for Junior ADUs. The specific number depends on foundation requirements, utility routing, and finish selections, all of which we itemize in the written quote. Call (669) 215-4852 for an exact quote on your property, estimates are free.
Yes. We install and warranty CWS, ES Windows, ECO Window Systems, Mr-Glass, and other major brands specified for addition and ADU projects. Our quotes name the manufacturer, model, and performance rating for every window and door, so the specification is verifiable before installation. Bring your preferred brand or let us recommend based on your project’s energy code requirements and exposure.
Home additions and ADU construction are planned projects, not emergency services, and we do not promise same-day dispatch for construction work. However, if you discover structural damage, water intrusion, or unsafe conditions during an existing project, we respond the same day to assess and secure the area. A live person answers our phones 24/7 to route urgent calls to the appropriate crew.
Every addition and ADU is covered by our 365-Day Done Right Promise: if it is not done right, Rowan makes it right, backed by The Haven Standard terms page, not a handshake. This applies to workmanship, materials, and installation. Manufacturer warranties on windows, fixtures, and appliances run separately and are transferred to you at project completion with registration assistance included.
Gather any existing surveys, site plans, or prior permit documents for your Mountain View property; note your approximate budget range and intended use for the new space; and list any specific concerns, such as drainage, tree preservation, or utility access. We bring measuring tools, a camera for the documented photo record, and sample material specifications. The estimate arrives in writing within 48 hours, with no obligation to proceed.
Schedule Your Home Additions & ADU Service in Mountain View Today
Call (669) 215-4852 to schedule your free estimate. We’ll visit your Mountain View property, assess feasibility, and deliver a flat written price with line-item scope before any work begins. That’s Clause 1 of The Haven Standard, and it’s been our practice since 2007. No obligation, no surprise add-ons, and a free second opinion on any competing quote you want reviewed.
Written by Grant Rowan, Owner at Rowan Construction Group, serving Mountain View since 2007.