Detached vs Attached Granny Flat — Which Is Better?

Two Building Types, Two Different Projects

Detached granny flat: standalone building, separated from the main house by at least 3 metres. Own walls, own roof, own services, own front door from the side path.

Attached granny flat: built onto the rear or side of the existing house. Shares a wall with the original dwelling. Separate entry, separate services (electrical, hot water, gas can all be metered separately), but the building envelope is continuous with the main house.

Both are allowed under the SEPP. Both can be CDC. Both rent. The decision matters because the cost, planning, and end-product feel are different.

Detached — What You Pay For

All-in cost: $150,000-$210,000 for a 60 sqm 2-bed.

What you're buying: • Complete acoustic separation. No shared wall = no noise transfer to the main house. • Independent feel for the tenant. Own outdoor area, own pathway, own bin storage. • Cleaner refinance valuation. Banks treat detached secondary dwellings closer to a 'separate dwelling' for the purposes of property valuation than attached. • Higher rent. Western Sydney 2026 data: detached 2-bed commands $20-$40/week more than an equivalent-spec attached unit.

What it costs: • 3m setback from the main house (eats backyard). • Longer service runs — sewer, water, electrical, NBN all need to travel from the existing house out to the new dwelling. On a block where the sewer main is 25-30m from the granny flat slab, that single trench can cost $12k-$18k. • Separate roof. No shared structure to absorb cost.

Best fit: blocks 550 sqm+, investment-focused, rental-first.

Attached — The Quiet Money-Saver

All-in cost: $120,000-$165,000 for a 60 sqm 2-bed. Roughly $25k-$45k cheaper than detached.

Where the saving comes from: • Shared wall (one less external wall = less framing, cladding, insulation). • Shared services run. The new sewer, water and electrical typically tee off the existing house plumbing within 2-3 metres of where they enter the main building. • No 3m setback from the main house — the building is the main house, just extended. • Sometimes a continuation of the existing roofline, reducing trade complexity.

What you give up: • Some acoustic transfer through the shared wall (we use double-stud + acoustic batts to mitigate, costs $1,500-$2,500 extra). • Tenant doesn't feel as 'independent'. Rent is $20-$40/week lower. • Some bank valuations treat attached units more like a renovation than a second dwelling, which can dampen the valuation uplift.

Best fit: blocks 450-550 sqm where 3m + 3m setbacks would consume the whole backyard. Elderly parent accommodation where proximity is wanted, not avoided.

Planning Trade-offs Most People Miss

Attached doesn't always equal cheaper approval. If the attachment changes the principal dwelling's footprint, the certifier may require updated BASIX on the WHOLE house (not just the new portion). That is a $800-$1,200 BASIX cost and 1-2 weeks of redrafting.

Detached on a battle-axe block can be a planning headache — the certifier has to confirm fire separation between the two buildings, which sometimes triggers BCA fire-rated wall requirements ($3,000-$5,000 extra).

Detached usually gives you more layout flexibility. Attached forces the granny flat layout to align with the existing house roofline and structural grid — sometimes this is fine, sometimes it ruins the floor plan.

If the existing house is older (pre-1990) and the attached unit ties into shared services, a service upgrade may be required (electrical board, sewer connection). Add $5k-$12k.

Which One We'd Build on Your Block

Block 550 sqm+, 2-bed for rent: detached. Block 450-549 sqm, tight setbacks: attached, almost always. Elderly parent + want connecting door: attached. Elderly parent + want independence: detached. Build now, sell duplex later: detached (preserves more options). House is pre-1980s with old services: detached. The cost of upgrading the original house services to support an attached granny flat often outweighs the saving from the shared wall.

Buildana's free site assessment includes a detached vs attached recommendation backed by costed comparison. Contact us.

If granny flat lands as the right call for your site, jump straight to the layouts at /homes/granny-flats/designs — 47 plans across 1, 2 and 3-bedroom configurations.