Internal ADU Guide: Types, Code & Process | Confido Builders

ADU Guide · Internal Conversions

Your home already has space for an ADU. Here's how to find it.

An internal ADU converts space that already exists inside your home — a basement, an attic, or an unused floor — into a fully independent living unit. No new footprint. No detached structure. Lower cost and simpler permitting than you'd expect.

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20–40%
Lower cost than a detached ADU, on average
900 SF
Maximum by-right ADU size in Massachusetts
4
Types of internal ADU conversions covered in this guide
$250K
Max MassHousing financing available at 5.25%
Internal ADUNo new footprint required
3–6 Month BuildAfter permits are approved
By-Right in Most MA TownsNo variance needed
$80K–$180KTypical cost range
9 Code Requirements780 CMR & 527 CMR
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Comparison

Two paths to an ADU. Very different projects.

Both create an independent living unit on your property. But internal and detached ADUs differ significantly in cost, complexity, permitting, and what they require from your site. Understanding the difference is step one.

This Guide
Internal ADU
Works within your existing building envelope
Alternative Path
Detached ADU
A new structure on your property
Cost
Typically $80K–$180K depending on scope and existing conditions
Typically $180K–$400K+ — new foundation, structure, utilities, and systems
Zoning & Permitting
By-right in most MA municipalities under the Affordable Homes Act. No variance needed in most cases.
Also by-right in most MA towns — but subject to setback, lot coverage, and height rules that may limit feasibility.
Site Requirements
No additional land needed. Works on any lot size as long as the existing building has convertible space.
Requires adequate lot depth, setbacks, and utility access. Smaller or irregular lots may not qualify.
Construction Timeline
3–6 months typical. No foundation, no weather delays on framing.
8–18 months typical. Full construction from ground up including site prep, foundation, and all systems.
Key Constraint
Existing ceiling heights, egress options, and building envelope determine what's possible. Not every home qualifies.
Higher upfront cost and longer timeline. Lot constraints can eliminate the option entirely on smaller properties.
Privacy & Character
Shares walls with main unit. Layout and sound transmission require thoughtful design. Best for trusted tenants or family.
Fully separate structure with own entrance, yard access, and complete acoustic separation. Maximum tenant independence.
Not sure which is right for your property? Our free site assessment covers both paths — we'll evaluate your existing building for internal conversion potential and your lot for detached feasibility, and give you a clear recommendation based on your goals and budget.

Four Paths

Every internal ADU starts with what you already have.

Internal ADUs work within your existing building. The right approach depends entirely on your home's layout, ceiling heights, and existing conditions. Here are the four primary types.

Most Common
grade MAIN UNIT ADU 7ft
01
Type 01 · Most Frequently Permitted
Basement Conversion

The most frequently permitted internal ADU in Massachusetts. An existing basement is finished, insulated, and brought up to code as a self-contained unit with its own entrance, kitchen, and bathroom.

Typical Cost $80K – $140K
Lowest cost entry point 6'8" min ceiling (existing) Backwater valve required Egress window per bedroom
ADU MAIN UNIT pitch
02
Type 02
Attic Conversion

Unused attic space is converted into a livable unit, typically accessed via a dedicated stair. Works best in homes with generous roof pitch. Dormer additions can expand usable square footage but add cost.

Typical Cost $100K – $180K
Roof pitch is critical Dedicated stair required Great natural light potential Dormers may trigger review
MAIN UNIT ADU entry entry
03
Type 03
Floor Split

A multi-story home is divided horizontally — typically ground floor and basement as the ADU, upper floors as the main unit. Creates a generously sized ADU with full-height rooms and complete mechanical separation.

Typical Cost $120K – $200K
Larger ADU footprint Full room heights Separate entrances required Mechanical separation needed
MAIN ADU
04
Type 04
Carve-Out from Existing Space

A portion of an existing floor is partitioned off to create a self-contained unit within the main home's footprint. The most flexible approach in terms of location, but typically produces smaller units.

Typical Cost $80K – $150K
Most layout flexibility Smaller unit size typical Sound separation critical Plumbing routing a challenge

Quick Qualifier

Can your home support an internal ADU? Here's what matters.

Before any design work, these are the four questions that determine whether an internal ADU is viable for your property. A free site visit from Confido answers all of them in one visit.

Does your basement have at least 6'8" of ceiling height?
For existing basement conversions, Massachusetts allows a minimum of 6 feet 8 inches under 780 CMR. New construction habitable space requires 7 feet. Many older basements fall short even of 6'8" — measure before planning anything else.
Can you add a code-compliant egress?
Every bedroom needs a window with a minimum 5.7 sq ft net clear opening, at least 20" wide and 24" high. The unit also needs two emergency egresses. Existing window openings often need enlarging.
Is your property zoned for an ADU?
Under the Massachusetts Affordable Homes Act, most municipalities must allow ADUs by-right on single-family lots. A few towns have specific restrictions — your municipality's zoning ordinance is the definitive source.
Can you create a separate entrance?
The ADU must have its own entrance visible from the street for emergency access. For basement conversions, this typically means a bulkhead door or a new exterior door cut into the foundation wall.
Can plumbing be extended to the new unit?
A kitchen and bathroom require water supply and drain lines. For basements, a backwater valve is required for any below-grade plumbing fixture. Routing complexity varies significantly by home layout.
Is there space for dedicated HVAC?
The ADU must have its own heating and cooling system — it cannot share the main unit's HVAC. A mini-split system is the most common solution for internal ADUs, requiring minimal ductwork and a small exterior unit.

Massachusetts Building Code

What every internal ADU must meet.

These requirements apply statewide to internal ADUs that don't add gross floor area to the building. They're set by the Massachusetts State Building Code (780 CMR) and Fire Code (527 CMR) — your municipality enforces them through the building permit process.

The most common project-stopper: Ceiling height. For existing basement conversions, 780 CMR allows a minimum of 6 feet 8 inches (new construction requires 7 ft). Many older basements fall short even of 6'8" — always confirm this first before any other planning begins.
Structural & Space Requirements
Common Issue
01
Living Area Ceiling Height
7 feet minimum for new construction habitable space; 6'8" minimum allowed for existing basement conversions under 780 CMR. The most common feasibility barrier — measure before planning anything else.
02
Common Access to Utilities
Unobstructed access to water, gas, and electric shut-offs must be maintained. Cannot be blocked by ADU partition walls or locked within the ADU.
03
Two Emergency Egresses
Neither exit can pass through a bedroom, bathroom, or utility room. Exits should be as far apart as possible. Primary egress must be visible from the street.
Fire & Life Safety
Common Issue
04
Bedroom Egress Windows
Each bedroom must have a window meeting egress requirements: minimum 5.7 sq ft net clear opening, at least 20" wide and 24" high, with a sill height no more than 44" from the floor.
05
Sprinkler System
Requirements scale with building size: 1-family + ADU: none required. 2-family + ADU: tank system (NFPA 13D). 3-family + ADU: wet line to street (NFPA 13R).
06
Heat Detection
2F and 3F buildings with sprinklers connected to city water and a dedicated fire line must install system-connected heat detectors in each non-ADU unit, monitored by a central station.
07
Hard-Wired Smoke & CO₂
All configurations — 1F, 2F, and 3F — require hard-wired (not battery) smoke alarms and carbon monoxide detectors in every unit throughout the building.
09
Emergency Response Access
The ADU's existence, location, and access point must be clearly identifiable to emergency personnel. Specific lighting, signage, and wayfinding are required at the entrance.
Plumbing
08
Backwater Valve
Any plumbing fixture installed in the basement requires a backwater valve to prevent sewage backup into the unit from main line surges — a common issue in older neighborhoods.
Note
Separate Utility Metering
While not always required, separate electric metering for the ADU is strongly recommended. It simplifies billing for rental units and is often required by lenders for financing programs.
How Confido handles this: We complete a full code feasibility review before any design work begins. Ceiling heights, egress options, sprinkler requirements, and utility configurations are assessed at the site visit — so there are no surprises mid-project.

From Idea to Keys

What it actually takes to get one built.

An internal ADU typically takes 6–12 months from first conversation to move-in. Here's what each phase involves and what you need to have in place at each step.

Total Timeline: 6–12 Months
1Feasibility
2Design
3Permits
4Construction
5CO
1
Phase One
Site Feasibility
2–4 weeks

Before any design work, confirm whether your home can support an internal ADU. The key questions: ceiling height, egress options, zoning compliance in your municipality, and which type of conversion makes the most sense for your layout and goals.

What we do: Confido's free site report covers all of this — zoning lookup, ceiling height assessment, egress analysis, and a clear recommendation on which ADU type fits your property. No cost, no obligation.
2
Phase Two
Design & Financing
4–8 weeks

An architect produces drawings for permit submission. Simultaneously, you explore financing — whether that's a HELOC, the MassHousing ADU Loan Program (up to $250K at 5.25%), or a combination. These two tracks run in parallel.

Financing options: MassHousing ADULP offers subsidized rates for owner-occupants statewide. MHP provides a free professional feasibility study. Boston homeowners have additional city programs layered on top. See full financing guide →
3
Phase Three
Permitting
4–10 weeks

Your contractor submits for a building permit through your municipality's building department. Internal ADUs typically require a full long-form permit. Timeline varies by town — plan for 4–10 weeks. Your contractor handles the submission; you handle the fee payment.

What to expect: The permit application includes your architect's stamped drawings, a site plan, and documentation of code compliance for the 9 requirements above. Your contractor will know exactly what each jurisdiction requires.
4
Phase Four
Construction
3–6 months

Internal ADU construction typically runs 3–6 months depending on scope. Most of the work is interior — framing, insulation, mechanical, electrical, plumbing, and finishes. Your home remains livable throughout in most cases, with some disruption during rough-in work.

Typical scope: Framing and partition walls · Egress window installation · Electrical panel upgrade or subpanel · Plumbing rough-in and fixtures · HVAC or mini-split installation · Insulation · Drywall, flooring, and finishes · Separate entrance if needed
5
Phase Five
Inspections & Certificate of Occupancy
2–4 weeks

Your building department inspects at key milestones — framing, rough mechanicals, insulation — and issues a final Certificate of Occupancy when the unit passes final inspection. The CO is what makes the unit legal to rent or occupy.

Don't skip this: An ADU without a CO cannot legally be rented and may cause complications when you sell your home. Unpermitted units also don't count toward your property's assessed value correctly and won't qualify for financing programs.

Find out if your home qualifies. It's free.

We assess your property for internal ADU feasibility — ceiling heights, egress options, zoning, and financing paths. You get a clear recommendation with no obligation.

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Building code requirements reflect Massachusetts 780 CMR and 527 CMR as of May 2026. Code requirements and local zoning rules vary by municipality and are subject to change. Always verify current requirements with your local building department. Confido Builders is a licensed Massachusetts general contractor — see our financing guide for state and city incentive programs.

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