Answer first: In NSW, a dual occupancy — or a new principal dwelling plus a new secondary dwelling on the same lot — typically starts on the BASIX multi-dwelling pathway when you are assessing more than one new dwelling together. Shared rainwater tanks, landscaping and pools are entered using Planning Portal apportionment rules (or as common areas and central systems when both dwellings are new). A granny flat alone usually follows the single-dwelling path; renovations use alterations and additions when works are valued at $50,000 and over and/or a pool and/or spa has a total volume greater than 40,000 litres.
Author: Franz Phan, Senior Planning Consultant, giantA Pty Ltd · Updated 22 September 2026
What is a dual occupancy for BASIX project-type purposes?
On the NSW Planning Portal, dual occupancy guidance sits under BASIX project details for project type. It covers attached or detached dwellings that share the same site. Two everyday scenarios matter most for certificates:
- One new dwelling on a site that already has an existing dwelling (creating a dual occupancy arrangement).
- Two new dwellings assessed together on the same project.
Those two cases do not always start with the same “Start new project” selection. The Portal dual occupancy help note is explicit:
- If you are building one new dwelling on a site with an existing dwelling: in the “Start new project” popup, select “1 dwelling”, then apportion any shared facilities using the Portal guidelines.
- If you are building two new dwellings: select “more than 1 dwelling”, and enter shared facilities as common areas and central systems.
That distinction is the root of many RFIs. Teams often treat every dual occupancy as identical in the tool when the Portal splits the start path by whether one or both dwellings are new.
Source: Dual Occupancy (BASIX project type).
When does the BASIX multi-dwelling pathway apply?
When you start a BASIX application, primary and secondary (and sometimes tertiary) selections lock the assessment pathway: single dwelling, multi-dwelling, or alterations and additions. The Portal states that once you start, the pathway cannot be changed.
From the “Starting a BASIX application” tables, multi-dwelling applies in combinations that include (among others):
| Primary selection | Secondary / tertiary (as applicable) | Pathway applied |
|---|---|---|
| One dwelling | Single lot and the lot includes common property and/or the project also includes a secondary dwelling | Multi-dwelling |
| One dwelling | The project involves sub-division or strata title (with or without the tertiary secondary-dwelling/common-property option) | Multi-dwelling |
| Two or more dwellings | At completion, each dwelling will be on a separate lot | Multi-dwelling |
| Two or more dwellings | Some or all dwellings combined on one lot (such as dual occupancy or residential flat buildings) | Multi-dwelling |
Practically: if your proposal is two new dwellings on one lot described as dual occupancy, the Portal tables point to multi-dwelling. If subdivision or strata is involved, do not force a single-dwelling start just because the architecture “feels like two houses”.
Source: Starting a BASIX application.
When do a new principal dwelling and a new secondary dwelling count as one multi-dwelling project?
Secondary dwelling (granny flat or bedsitter) help notes define a secondary dwelling as a self-contained dwelling established in conjunction with a principal dwelling on the same lot, located within, attached to, or separate from that principal dwelling. If there is an intention to further subdivide or strata title, the Portal notes the project is not a secondary dwelling for that purpose.
For BASIX classification, the secondary-dwelling help note groups common situations as follows:
| Situation | BASIX tool / pathway guidance (Portal) |
|---|---|
| Conversion of habitable space within the footprint of an existing principal dwelling | Alterations and additions |
| Conversion of existing non-habitable space (e.g. garage, shed); conversion of a non-residential building into a secondary dwelling; or construction of a new secondary dwelling with an existing principal | New single-dwelling tool (with apportionment for site/landscape where noted) |
| Construction of a new principal and a new secondary as one project | New multi-dwelling tool — enter like a dual occupancy (more than one dwelling on a single lot) |
The December 2025 BASIX Newsletter reinforces the single-dwelling point for an existing principal plus a new granny flat or conversion of non-habitable space: assess that new development as a new single dwelling house, not as alterations and additions to an existing BASIX building. New-dwelling standards apply in those cases.
So the “one project” multi-dwelling call is specifically when both principal and secondary are new and lodged together — not when you are only adding a granny flat beside an existing house.
Sources: Secondary Dwelling (Granny Flat or Bedsitter) · BASIX Newsletter December 2025.
Related giantA reading for granny-flat-only energy compliance: Granny Flat Energy Compliance NSW 2026.
How do shared rainwater tanks, landscaping and pools get entered?
For dual occupancy arrangements where you are completing assessments that require apportionment, the Portal dual occupancy guideline applies to attached or detached dwellings that share the same site. If the dwellings share any of the following, you must apportion before entering values for each dwelling:
- landscaping
- rainwater tank
- stormwater tank
- swimming pool or spa
- alternative energy supply
Apportionment uses an INPUT FACTOR from the Portal’s bedroom-number table, multiplied by the area, volume or power of the shared item. The Portal’s worked example (3-bedroom and 4-bedroom dwellings sharing a 7,000-litre rainwater tank) uses INPUT FACTORS of 0.44 and 0.56, giving apportioned tank volumes of 3,080 litres and 3,920 litres respectively. Always read the live Table 1 on the Portal for the bedroom combination that matches your project — do not invent factors.
High-level entry rules from the same help note include:
- Site area: enter the site area of the whole site.
- Roof area: if roofs are separate, enter each dwelling’s roof; if shared (for example garage roofs), add the shared roof to each separate dwelling’s roof area as the figures illustrate.
- Landscaping: separate landscape areas enter per dwelling; shared landscape areas are multiplied by the INPUT FACTOR.
- Rainwater / stormwater tanks: shared tanks require apportioned collection areas and tank sizes using the INPUT FACTOR.
- Swimming pool and spa (water): shared volume is multiplied by the INPUT FACTOR for each dwelling’s entry.
- Thermal comfort: each dwelling can be described separately without an INPUT FACTOR.
- Hot water (shared): both dwellings must enter the hot-water system type separately.
- Pool/spa energy questions: if sharing a pool/spa, both dwellings answer timer and heating questions separately.
- Shared PV: multiply system kW by the INPUT FACTOR for each dwelling.
When you are building two new dwellings and selected “more than 1 dwelling”, the dual occupancy note instead directs you to enter shared facilities as common areas and central systems — a different data-entry pattern from single-dwelling apportionment. Match the entry method to the start selection you actually used.
Source: Dual Occupancy — apportionment guidelines.
How does this differ from the alterations and additions pathway?
Alterations and additions is a separate pathway. The Portal alterations page states that BASIX applies to all alterations and additions in NSW that are valued at $50,000 and over and/or involve the installation of a pool and/or spa with a total volume greater than 40,000 litres.
The “Starting a BASIX application” table maps those triggers to the alterations pathway, including:
- Alteration or addition → estimated development cost $50,000 or more
- Alteration or addition → pool and/or spa with combined size of 40,000 litres or more
- New pool or spa → combined size 40,000 litres or more, or other work costing $50,000 or more
Do not use outdated informal renovation thresholds. Live Portal pages and the December 2025 newsletter use $50,000 — not older figures you may still see in secondary commentary.
Also do not force an alterations path onto a new granny flat beside an existing house when the Portal and newsletter say that case is assessed as a new single dwelling. Mis-routing new dwellings into alterations is a common certificate and council query.
Sources: Alterations and additions · About BASIX.
Related giantA reading: When your renovation triggers BASIX · Why you still need a certificate over $50k · Pools, spas and the 40,000-litre trigger.
What should you decide before clicking “Start application”?
- Count how many dwellings are new in this application — one new dwelling beside an existing house is not the same start path as two new dwellings.
- Confirm whether principal + secondary are both new — if yes, the secondary-dwelling help note points to the multi-dwelling tool as for dual occupancy.
- List shared facilities (landscape, tanks, pool/spa, PV, hot water) and decide whether you will apportion (one-new-dwelling dual occupancy pattern) or enter common/central systems (two-new-dwellings pattern).
- Check subdivision or strata intent — Portal tables push those combinations to multi-dwelling; secondary-dwelling classification also changes if subdivision/strata is intended.
- Separate renovation scope that might sit on alterations ($50,000+ and/or pool/spa > 40,000 L) from new-dwelling scope so you do not collapse two pathways into one incorrect start.
- Align NatHERS / thermal comfort per dwelling — dual occupancy notes allow separate thermal comfort without INPUT FACTORS, and drawings must match shared-facility commitments.
For certificate timing after generation, see BASIX certificate validity and the 3-month window. Where alterations modelling is complex, see BASIX simulation / NatHERS for alterations over $50k.
What lodgement pitfalls are common on dual occupancy BASIX?
- Starting “1 dwelling” when both dwellings are new — or the reverse — against the dual occupancy help note.
- Treating new principal + new secondary as alterations instead of multi-dwelling.
- Treating a new granny flat beside an existing house as alterations when Portal/newsletter guidance points to new single dwelling.
- Entering full shared tank or pool volume on both dwellings without INPUT FACTOR apportionment when the one-new-dwelling dual occupancy pattern applies.
- Ignoring common/central systems entry when the two-new-dwellings start path was selected.
- Using outdated alterations figures — live Portal threshold is $50,000 and over (never older informal amounts).
- Changing pathway mid-stream — you cannot; restart with correct selections.
How should architects sequence dual occupancy BASIX with DA or CDC?
Decide dwelling count and shared-services strategy at concept stage. Shared tanks, landscape and pools affect BASIX inputs and engineering/landscape sets. If alterations to an existing dwelling independently hit $50,000 or a >40,000-litre pool trigger, clarify with your certifier whether those works need a separate alterations certificate. BASIX does not choose DA versus CDC — confirm the planning pathway, then lodge certificates that match stamped drawings. Re-check live Planning Portal pages before lodgement.
How does giantA help with dual occupancy and multi-dwelling BASIX?
giantA Pty Ltd provides architecture, drafting and BASIX/NatHERS support from Parramatta CBD across Greater NSW. We help dual occupancy and principal-plus-secondary teams choose the correct Portal start path, enter shared facilities consistently with drawings, and coordinate thermal comfort / NatHERS inputs per dwelling before lodgement. Explore BASIX and NatHERS, our services, or contact giantA for a project-specific quote — we do not publish fixed fees in blog posts.
FAQ: BASIX dual occupancy and multi-dwelling pathways
Is every dual occupancy automatically multi-dwelling in BASIX?
Not always. Portal dual occupancy guidance splits one new dwelling beside an existing dwelling (start as 1 dwelling + apportionment) from two new dwellings (more than 1 dwelling + common/central systems). Starting-application tables also map two-or-more dwellings on one lot to multi-dwelling. Match your facts to the live tables.
When do principal + secondary count as one multi-dwelling BASIX project?
When both are new and constructed as one project, the secondary-dwelling help note says to use the new multi-dwelling tool in the same way as dual occupancy (more than one dwelling on a single lot).
Is a new granny flat beside an existing house multi-dwelling?
Usually no. Portal and December 2025 newsletter guidance treat that as a new single dwelling assessment (not alterations), with apportionment notes for site/landscape where applicable.
How do I enter a shared rainwater tank?
Under the dual occupancy apportionment guideline for applicable one-new-dwelling arrangements, multiply roof collection area and tank size by the INPUT FACTOR for each dwelling. For two new dwellings started as “more than 1 dwelling”, enter shared facilities as common areas and central systems per the same help note.
What is the alterations cost threshold in 2026?
$50,000 and over on live Planning Portal alterations pages and the December 2025 newsletter. Do not use outdated informal amounts.
Do pools still matter on dual occupancy sites?
Yes. Shared pool/spa volume is apportioned under dual occupancy guidelines where that pattern applies. Separately, pools/spas with total volume greater than 40,000 litres (Portal wording also uses “40,000 litres or more” in start tables) can trigger the alterations pathway for relevant scopes.
Can I change the pathway after I start?
No. The Portal states the assessment pathway cannot be changed once you start the application.
Where should I start if I only know “dual occ or granny flat”?
Count how many dwellings are new, confirm subdivision/strata intent, list shared facilities, then map Portal primary/secondary selections. Read this with giantA’s alterations trigger guide and the live Portal pages.
This article is general information for NSW homeowners, builders and designers. It is not legal advice or a site-specific BASIX assessment. Confirm the current Planning Portal BASIX pages, Sustainable Building SEPP, and advice from a suitably qualified practitioner before acting. Cited NSW Planning Portal sources researched 22 September 2026.