Last researched: September 19, 2026. Once your ADU permits are issued, the project changes from paperwork to sequencing. The contractor now has to coordinate site work, foundation, framing, roofing, plumbing, electrical, HVAC, insulation, drywall, finishes and inspections in an order that allows each hidden stage to be checked before it is covered.
This guide starts after permit issuance. If you are still working through zoning, plans or permit review, use our Maryland ADU Permits to Completion guide first.
Maryland ADU construction sequence
Pre-construction handoff
Before excavation or demolition, confirm that the contractor is working from the approved plan set and current permit documents. Check who is responsible for scheduling inspections, utility coordination, erosion/sediment controls, temporary power, dumpsters, material storage and site access.
Anne Arundel County’s current inspection guidance states that approved construction drawings and grading plans must remain on site during construction. Other jurisdictions may have similar requirements.
Do not start work that requires a permit before the permit is active.
Site preparation and layout
For a detached or attached ADU, the contractor typically establishes the building location, protects required areas and prepares for excavation. This is where conflicts with actual grade, utilities, easements, trees or access sometimes appear.
If the footprint differs from what was approved, stop and resolve it before the foundation locks the project into the wrong location. Compare the field layout with the approved site-plan constraints.
Excavation, footing and foundation
The next work may include excavation, footing forms/reinforcement, foundation walls, slab preparation, drainage, waterproofing and backfill. The exact system depends on the design.
Do not cover inspection points prematurely. Anne Arundel’s current directory, for example, lists separate building inspection stages for footing, backfill, draintile and slab.
Inspection gate: footing / foundation-related inspections as required locally.
Underground plumbing and utilities
Plumbing below slabs, water/sewer lines, electrical trenching and other underground work must be coordinated before those areas are covered. Utility work is a common dependency because a building can be framed quickly but still be delayed by service upgrades, trench routes or connection approvals.
Anne Arundel currently lists plumbing ground inspection, electrical trench inspection and water/sewer inspection categories separately.
Inspection gate: underground plumbing / trench / utility inspections where applicable.
Framing and dry-in
Once the foundation is ready, framing establishes floors, walls and roof structure. Sheathing, windows, exterior doors and roofing move the project toward “dry-in,” meaning the structure can better resist weather while interior work continues.
Keep framing accessible until required structural and trade rough inspections are complete. Closing walls too early can force removal of finished work.
MEP rough-ins: plumbing, electrical and HVAC
Mechanical, electrical and plumbing trades install pipes, wiring, boxes, ducts, equipment connections and other components that will later be hidden behind walls or ceilings.
These trades often need separate permits and inspections. Anne Arundel currently lists plumbing rough-in, electrical rough-in and mechanical rough-in as distinct inspection types. Montgomery County’s scheduling system similarly notes that applicable electrical and mechanical rough/close-in inspections must be coordinated with framing/close-in inspection.
Inspection gate: building/framing plus applicable trade rough-ins.
Insulation and air-sealing stage
After rough inspections are accepted, insulation and required air-sealing work can proceed. This stage matters for comfort and energy performance, especially in detached units with a large exterior surface area.
Anne Arundel currently lists insulation as its own building inspection type. Do not assume the inspector will accept insulation that hides work needing a prior rough inspection.
Inspection gate: insulation / energy-related inspection as required.
Drywall and interior close-up
Drywall changes the project quickly because much of the rough work disappears from view. After hanging, taping and finishing, the sequence moves toward cabinets, trim, flooring, tile, paint and fixtures.
This is why the rough-inspection stage is one of the most important schedule gates. Missing an inspection before close-up can create avoidable rework.
Exterior finishes and site restoration
Siding, exterior trim, gutters, decks/steps, walkways, grading and landscaping may overlap with interior finish work. Final grading and drainage should match the approved plans and should not create water problems around the new unit or neighboring property.
Fixtures, equipment and finish trades
Plumbing fixtures, electrical devices, lighting, HVAC equipment, appliances, cabinets, countertops and finish carpentry are completed. Contractors also work through their own punch lists before calling for final inspections.
Do not treat “looks finished” as “ready for occupancy.” The project still needs required trade and building finals.
Trade final inspections
Electrical, plumbing, mechanical and other applicable trade permits move through final inspection. Anne Arundel’s current guidance says all associated trade permits must pass their final inspections before the final building inspection can be scheduled.
Inspection gate: trade finals.
Final building inspection and lawful occupancy
The final building inspection checks the completed work against the approved plans and applicable code requirements. Resolve correction notices and obtain whatever final approval, certificate or occupancy documentation your jurisdiction requires before anyone moves into the ADU.
Anne Arundel’s contractor guidance expressly states that occupancy is prohibited until final inspections are completed.
Final gate: do not occupy before required final approval.
Where the construction schedule usually slips
A trade finishes, but the next inspection cannot happen immediately or another prerequisite inspection was missed.
Service upgrades, sewer/water connections, trenching or utility-company scheduling can sit outside the builder’s normal production flow.
Unexpected soil, drainage, buried utilities, rock, foundation conditions or septic conflicts can change the scope.
Windows, electrical equipment, HVAC components, cabinets or specialty doors can hold up downstream trades.
Failed inspections create rework and another inspection cycle. Anne Arundel notes that reinspection fees can apply when work is not ready or violations remain uncorrected.
Moving walls, changing windows, switching fixtures or revising finishes after construction starts can affect permits, purchasing and trade scheduling.
What can overlap, and what should not
| Work | Can often overlap with | Do not get ahead of |
|---|---|---|
| Exterior dry-in | Early interior rough work | Required structural inspection points |
| Plumbing/electrical/HVAC rough | Each other when coordinated | Framing access and rough inspections |
| Exterior finishes | Drywall/interior finishes | Required exterior/weatherproofing inspections |
| Cabinets/trim/paint | Some exterior site work | Wall close-up prerequisites |
| Landscaping | Late interior finish work | Heavy equipment, final utility work and grading |
| Move-in | Nothing | Required final approvals |
Detached ADU vs conversion timeline
A detached ADU usually has more site and foundation work because it creates a new structure. A garage or basement conversion can remove some of those phases, but it introduces a different risk: the existing structure may need repairs or upgrades before it can function as habitable space.
That is why a conversion is not automatically “faster.” A garage with an unsuitable slab, poor wall assembly or utility limitations can require substantial reconstruction. A basement may need moisture work, egress changes or structural modifications. Use the garage conversion guide or basement ADU guide before assuming the existing shell saves time.
Homeowner construction-control checklist
- Keep a copy of the approved plans and permits accessible on site.
- Know who schedules every inspection.
- Track inspection results and correction notices in one place.
- Do not allow walls, slabs or trenches to be closed before required inspections.
- Keep change orders written, priced and approved before work proceeds.
- Track long-lead items separately from ordinary materials.
- Ask the contractor which activity is currently on the project’s critical path.
- Photograph concealed plumbing, wiring and framing before drywall, where permitted and safe.
- Keep final lien releases, warranties, manuals and inspection records together.
- Do not allow occupancy until the jurisdiction’s required final approval is complete.
A better way to ask your contractor about timing
Instead of asking, “How many months will this take?”, ask for a dependency-based schedule:
- What must happen before foundation work begins?
- Which inspections can stop the next trade?
- Which materials have the longest lead time?
- Which utility work depends on an outside agency?
- When are owner selections needed to avoid delays?
- What conditions would trigger a plan revision or change order?
- What has to pass before drywall?
- What has to pass before final building inspection?
This produces a more useful schedule than a single promised completion date.
Maryland ADU construction timeline FAQs
How long does it take to build an ADU in Maryland?
There is no reliable statewide duration. Construction time depends on ADU type, site work, utilities, inspections, contractor capacity, materials, weather and corrections. A property-specific schedule is more useful than a generic number.
What inspections happen during ADU construction?
The exact list depends on the jurisdiction and project. Typical categories can include footing/foundation, slab, framing, insulation, plumbing rough/final, electrical rough/final, mechanical rough/final and final building inspection. Confirm the required sequence on your issued permits.
Can drywall go up before rough inspections?
Do not conceal work that still requires inspection. Framing and applicable plumbing, electrical and mechanical rough work generally need to be inspected before walls and ceilings are closed.
Can I move into the ADU after construction is physically finished?
Not merely because the work appears complete. Obtain the required final inspections and occupancy approval from the jurisdiction first.
What causes the biggest ADU construction delays?
Common causes include inspection sequencing, utility coordination, unforeseen site or existing-building conditions, correction notices, long-lead materials and owner-requested changes.
Primary sources checked
- Anne Arundel County — Inspection Types Directory
- Anne Arundel County — Building Inspections
- Anne Arundel County — Trade Inspections
- Montgomery County — Inspection Scheduling Requirements
- Frederick County — Residential Construction Plan Requirements
Maryland ADU Guide is an independent research publication, not a permitting agency, architect, engineer or contractor. Inspection sequences and permit requirements vary by jurisdiction and project. Follow the issued permit documents and instructions from the authority having jurisdiction over your property.
What to do next
Start by finding the office that reviews your property. A mailing city or a county guide does not establish your parcel’s authority.
- Find Who Reviews My Property.
- Obtain your parcel identifier, survey, floor plans and utility records.
- Ask the reviewing office which current rules, supporting documents and approvals apply.
- Continue to your planning workspace.
Enable JavaScript to prepare a downloadable next-step brief. Planning guidance is not permit approval.