Prefab vs. Custom ADU: Pros, Cons, and Costs
Which Option Is Right for You in Massachusetts?
As Accessory Dwelling Units (ADUs) become more popular across Massachusetts, homeowners face a key decision early in the planning process:
👉 Should you build a prefab ADU or go the custom route?
Both have their advantages — but they come with different timelines, budgets, and flexibility. In this post, we’ll break down the pros, cons, and cost differences to help you make the right choice for your property.
🔹 What’s the Difference?
Prefab ADU
Also known as modular or factory-built ADUs, prefab units are built off-site, delivered in sections (or fully assembled), and installed on your property.
Custom ADU
A site-built ADU that is designed and constructed from scratch to match your property layout, style, and specific needs.
âś… Pros of Prefab ADUs
- Speed
Factory construction can happen in parallel with site prep — reducing total project time by 30–50%. - Cost Predictability
Prefab ADUs typically come with fixed package pricing, which reduces budget surprises. - Quality Control
Built in a controlled environment with inspections at every stage. - Streamlined Process
Many prefab companies handle permitting, delivery, and installation.
❌ Cons of Prefab ADUs
- Limited Customization
You choose from set floorplans, layouts, and finishes. - Delivery Logistics
Narrow lots, steep driveways, or utility wires can make delivery tricky or even impossible in some neighborhoods. - Permitting Still Required
Even prefab ADUs need city approval, inspections, and local tie-ins (plumbing, electric, foundation).
âś… Pros of Custom ADUs
- Fully Tailored to Your Property
Build around your landscape, home style, and personal preferences. - Flexible Design Options
Add a larger kitchen, multiple bedrooms, vaulted ceilings, or unique finishes. - Better Integration with Existing Structures
Especially ideal for attached ADUs, basement units, or seamless additions.
❌ Cons of Custom ADUs
- Longer Timelines
Design + permitting + construction = often 9–14 months total. - Higher Costs
Custom often means more labor, site work, and variable expenses. - More Coordination Required
You’ll need to manage (or hire someone to manage) architects, engineers, and contractors — unless you work with a full-service firm like ADULink.
đź’° Cost Comparison
| Type | Starting Cost | Typical Range |
|---|---|---|
| Prefab | ~$135,000 | $135K – $200K+ |
| Custom | ~$150,000 | $150K – $250K+ |
💡 Note: Costs vary based on unit size, finishes, city fees, and site conditions. We’ll provide a detailed quote after your free consultation.
đź“Ť Which Is Right for You?
| Criteria | Go Prefab If… | Go Custom If… |
|---|---|---|
| Speed matters | You want it done ASAP | You’re okay waiting 9–12 months |
| Budget control | You want predictable pricing | You have flexibility to invest more |
| Design freedom | You’re okay with pre-set designs | You want something fully unique |
| Lot complexity | You have easy access + flat ground | You need to fit into tight or odd-shaped lot |
🎯 Final Thoughts
Whether you go prefab or custom, ADULink is here to make the process seamless. We’ll help you:
âś… Choose the right path
âś… Get city approvals
âś… Manage every step from design to move-in
đź’¬ Want Help Deciding?
👉 [Book Your Free ADU Consultation]
Let’s talk about your goals, your property, and what makes the most sense for your timeline and budget.
