Accessory dwelling units give you flexible space for family, aging in place, or long-term rental income. In Hollis, two rulebooks shape what you can build and how you can use it: New Hampshire’s state ADU law and the Town of Hollis Zoning Ordinance. This guide breaks down the three pillars most owners ask about in plain English: sizing, parking, and rentals. You will also see how permits work and what changed in 2025.
Important note for 2025: New state ADU amendments took effect July 1, 2025. Where local rules conflict with the statute, state law controls. You will find links throughout so you can verify details directly in the official sources.
Why ADUs matter for homeowners
Accessory dwellings can solve real needs. An ADU can house a parent, give a recent grad a soft landing, or unlock steady rental income. The right plan can also boost property value and make your lot work harder without moving.
In Hollis, feasibility comes down to a few key questions:
- How big can you make the ADU and where can it go on the lot?
- How many parking spaces do you need and how must cars access the unit?
- Can you rent it short term or long term, and do you need to live on site?
This guide answers those questions and highlights the 2025 state-law updates so you can plan with confidence.
Plan your ADU: types and uses
Attached vs. detached structures
- Attached ADU: Shares at least one wall with the main home. It can feel more connected, may cost less per square foot, and often fits within the existing footprint or a small addition. Hollis’s ordinance expects attached ADUs to be designed so they can be re-incorporated into the main dwelling and to maintain interior access through a common wall see Town ordinance. State law allows attached ADUs by right where single-family homes are permitted RSA 674:72.
- Detached ADU: A separate structure like a cottage or over-garage unit. Hollis permits detached ADUs on lots that are at least twice the minimum lot size for the zoning district Town ordinance. State law also requires municipalities to allow at least one ADU and to permit detached ADUs in districts that allow single-family homes RSA 674:72.
New build vs. conversion
- Conversion: Repurposing space you already have, such as a basement, attic, or garage. This can shorten timelines and reduce cost. Conversions still trigger building code and may require septic evaluations if bedroom count increases.
- New build: Ground-up cottages or over-garage suites add flexibility and privacy. Expect more site work, design, and permitting steps.
Primary home and lot fit
Before you draw plans, look at:
- Lot size and zoning district to confirm detached ADU eligibility.
- Topography and drainage for placement and utility runs.
- Driveway layout for safe access and turning space.
- Existing septic capacity or potential for upgrades. Hollis requires certification that the system can handle added load, and state rules may require NH DES approval if load increases RSA 485-A:38 guidance.
Size, height, and placement standards
Maximum square footage and ratios
Here is the most important sizing update for 2025:
- State baseline sizing: Municipalities cannot restrict ADU living space to less than 750 square feet, and the default maximum is 950 square feet unless the municipality authorizes more RSA 674:72. This applies statewide as of July 1, 2025.
- Hollis local text: The May 20, 2025 ordinance lists a 300 to 800 square foot range and a cap that limits attached ADUs to 30 percent of the main home’s heated above-grade floor area Town ordinance.
Because state law controls when there is a conflict, plan around the state’s 750 to 950 square foot band unless and until the Town adopts a compliant update. If your design needs more than 950 square feet, ask the Town whether a higher maximum has been formally authorized.
Practical design tip: Focus on livability. A 750 to 950 square foot one- or two-bedroom layout can fit a full kitchen, laundry, storage, and accessible bath while meeting code and budget.
Setbacks, lot coverage, and height
While the ADU section focuses on use and size, placement is still governed by your zoning district’s base rules. Confirm setbacks, height, and lot coverage before you commit to a footprint. For detached units in particular, check whether accessory structure standards or any site plan review threshold apply Town ordinance.
Design, entries, and privacy
- Entries: State law no longer forces an interior door standard and protects your choice of ingress and egress for attached ADUs RSA 674:72. Hollis’s ordinance still references internal access for attached units. If a conflict arises in practice, ask the Building Department how they are applying the updated statute.
- Privacy: Use thoughtful window placement, fencing, and landscaping to reduce sightlines into neighboring yards.
- Architecture: Keep exterior finishes and rooflines compatible with the main house to support neighborhood fit and smoother approvals.
Parking, access, and utilities rules
On-site spaces and counts
- State baseline: If parking is required for the main home, a town may require up to one additional space for the ADU. It cannot impose more burdensome parking than for the principal dwelling. Required spaces can be on-site or at a legally dedicated off-site location at the owner’s discretion RSA 674:72.
- Hollis local text: The Town requires “adequate off-street parking” and enough room for ingress, egress, and turning. There is no fixed number in the ADU section, so staff will evaluate your site plan case by case Town ordinance.
Plan to show where cars will park and turn without backing into a busy road. Stripes and clear wayfinding help if you share a driveway.
Driveways, access, and safety
Keep sight lines clear near the street. Provide lighting that is bright enough for safety and wayfinding but shielded to avoid glare. Emergency access should be obvious, with unit addressing visible from the road.
Separate meters and services
- Water and sewer or septic: Municipalities cannot force a separate septic system, but you must prove adequate capacity. If the ADU increases flow, you will need approvals and possibly a system upgrade under state rules RSA 485-A:38 and NH DES guidance. Hollis requires septic certification by a licensed designer or engineer as part of the application Town ordinance.
- Power and gas: Separate meters are optional. Many owners share service to control costs, while investors sometimes prefer separate metering for cleaner billing.
Rental and occupancy limitations
Short-term vs. long-term rentals
- Hollis prohibits short-term rentals of ADUs that are less than four weeks. Transient use such as hotels, motels, rooming or boarding houses is not allowed for ADUs. If you want to host short stays, that falls under different local rules and is not permitted through the ADU pathway Town ordinance.
- State statute does not create a blanket right to short-term-rent an ADU. Local transient occupancy rules can still apply so long as they do not conflict with state law RSA 674:72.
Plan for leases of four weeks or longer if you intend to rent an ADU in Hollis.
Owner occupancy and residency
Hollis requires owner occupancy of either the principal unit or the ADU, with a hardship exception process in the ordinance. The state statute allows a municipality to require owner occupancy of one unit, but not to specify which one RSA 674:72 and Town ordinance.
This rule can affect financing and your operating plan, so decide early which unit you plan to occupy if you will rent the other.
Lease terms and compliance
Use written leases that define parking, trash, and quiet hours. Follow fair housing laws and keep the ADU well maintained. Good neighbor policies reduce complaints and protect your investment.
Permits, reviews, and project timeline
Pre-application and feasibility
- Confirm your zoning district, setbacks, and whether your lot qualifies for a detached ADU under the Town’s “twice the minimum lot size” rule Town ordinance.
- Verify septic capacity early with a licensed designer. If load increases, you may need NH DES approval before permits or occupancy NH DES.
- Sketch a simple site plan that shows building placement, parking, turning, and utility runs. A quick consult with the Building Department can save time Town Building/Code Enforcement.
Submittals, fees, and approvals
Typical packages include a site plan, floor plans, elevations, septic certification, and accurate square footage calculations. Hollis issues building permits and requires a Certificate of Occupancy before move-in. If your project triggers Planning Board review due to site changes, allow extra time Town ordinance.
Remember the 2025 state-law updates:
- One ADU is allowed by right in any zoning district that permits single-family homes, and detached ADUs must be permitted too RSA 674:72 and related amendments and HB 577 overview.
- Sizing floors and ceilings now default to 750 to 950 square feet unless the Town has properly authorized a different maximum RSA 674:72.
Inspections and final compliance
Expect footing, framing, electrical, plumbing, and final inspections. The Town will confirm that the ADU matches the approved plans and that septic and life-safety items are in place. Keep your approvals and septic documents for future refinancing or sale.
Move forward with confidence
Here is the bottom line for Hollis in 2025:
- Size: Plan around the state’s 750 to 950 square foot band unless Hollis adopts a compliant update. The Town’s earlier 300 to 800 language conflicts with current state law, which controls RSA 674:72 and Town ordinance reference.
- Parking: Show “adequate” off-street parking. The Town will review your plan, and state rules allow at most one additional required space for the ADU if parking is already required for the main home RSA 674:72 and Town ordinance.
- Rentals: No short-term stays under four weeks. Owner occupancy of one unit is required in Hollis. Long-term leasing is allowed within those rules Town ordinance.
Thinking about value, design, or timing an ADU with a future sale or refinance? We can help you model ROI, position the property, and connect you with local designers and septic pros. Start with a quick pricing check and feasibility chat. Get started with Christensen Group, Inc. and Get a Home Valuation.
FAQs
What is the legal ADU size in Hollis right now?
- As of July 1, 2025, state law sets a minimum of 750 square feet and a default maximum of 950 square feet unless the Town authorizes more. Hollis’s earlier 300 to 800 figures conflict and are overridden where inconsistent RSA 674:72 and Town ordinance reference.
Can Hollis require more than one extra parking space for an ADU?
- No. If parking is required for the main home, the Town may require up to one additional space for the ADU. Hollis uses an “adequate off-street parking” standard and will review your plan for safe access and turning RSA 674:72 and Town ordinance.
Are short-term rentals allowed for ADUs in Hollis?
- No. ADUs may not be used for transient stays under four weeks. Long-term leases are allowed within owner-occupancy rules Town ordinance.
Do I need to live on the property if I rent the ADU?
- Yes, Hollis requires owner occupancy of either the main home or the ADU, with a hardship exception process detailed in the ordinance Town ordinance and supported by state statute allowing owner-occupancy requirements RSA 674:72.
Is an interior door required for an attached ADU?
- Hollis references internal access for attached ADUs, but state law protects your choice of ingress and egress. If requirements conflict in practice, consult the Building Department on current enforcement under the 2025 statute RSA 674:72 and Town ordinance.
Can I build a detached ADU on my lot?
- Possibly. Hollis allows detached ADUs on lots that are at least twice the minimum lot size for your zoning district. State law also requires that at least one ADU be allowed by right, and detached units must be permitted in districts with single-family homes Town ordinance and RSA 674:72.
What septic steps are required?
- You must show adequate capacity. If bedrooms or flow increase, you may need NH DES approval and a system upgrade. Hollis requires a licensed designer’s certification with your application NH DES and Town ordinance.
Where do I start with permits?
- Begin with a feasibility check and a quick consult with Building/Code Enforcement, then submit plans for a building permit. A Certificate of Occupancy is required before move-in Town Building/Code Enforcement and Town ordinance.
