St Marys has many established properties where a granny flat or secondary dwelling may be possible, with older streets commonly featuring 550–700m² blocks. The suburb is within Penrith City Council and is mainly zoned R2 Low Density Residential, with R3 and R4 pockets around the centre and station. Under the NSW Housing SEPP, a secondary dwelling may be eligible for complying development on a lot of at least 450m², provided the proposal meets all relevant controls, including the 60m² floor area limit. However, lot size alone does not guarantee approval, and properties within flood planning areas around South Creek and Byrnes Creek may have additional requirements. Each property needs to be assessed individually before a granny flat design is finalised.
Get inspiration from our recent granny flat projects across St Marys and surrounds — and see how we’ve turned different properties into practical new spaces.
St Marys is an established Western Sydney suburb where a granny flat can give homeowners more flexibility as family and housing needs change. The suburb is already well connected through St Marys Station, while the new Sydney Metro Western Sydney Airport connection and Western Sydney International Airport are bringing further investment, jobs and housing demand to the area. For homeowners, this creates a practical opportunity to make better use of an existing block by adding separate accommodation for ageing parents, adult children or extended family, while retaining the main home for the household. A well-designed granny flat can also provide potential rental income where the property and intended use are suitable, giving the home another layer of long-term flexibility. With St Marys Town Centre planned for substantial growth, adding a secondary dwelling can be a sensible way to use existing land more effectively while preparing the property for changing family and housing needs.
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The three things that shape a granny flat build in St Marys: the planning rules, the ground you’re building on, and what the market gives back.
Median house price in St Marys
Typical granny flat rent
Key local landmark
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nearest station
A granny flat in St Marys can be approved through a private certifier as complying development when the lot is 450m2 or larger, the secondary dwelling is no more than 60m2 (or 25% of the main house, whichever is less), and setbacks, height and landscaping meet Housing SEPP 2021 standards. This pathway typically takes 10 to 15 business days once documentation is complete. Blocks inside a flood planning area can still qualify, but floor levels and fill must meet the council's flood controls, which we check against the flood certificate before design.
Not meeting one complying development requirement does not rule a project out. Where a block in St Marys is under 450m2, sits in a heritage conservation area, or is affected by a flood planning control, a development application can be lodged with Penrith City Council. Assessment times are longer than a CDC, but the council does approve secondary dwellings through this route.
St Marys is administered by Penrith City Council and assessed under the Penrith LEP 2010. The whole suburb falls within the one council area. Granny Flats X confirms the consent authority for your specific block during the initial property assessment.
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The things St Marys homeowners ask us most. Can’t find yours? Call us — we’ll give you a straight answer.
Yes, a 60m² granny flat can be possible in St. Marys, subject to your block and the relevant planning requirements. A 60m² layout can work well as a 2-bedroom granny flat, depending on the design and available space. Our free site consultation can help determine what will work best for your property.
Check our actual 2-bedroom granny flat designs.
In St Marys, a granny flat typically costs around $120,000–$145,000 for a studio or 1-bedroom, $145,000–$180,000+ for a 2-bedroom, and $175,000–$220,000+ for a 3-bedroom granny flat in 2026. These are indicative price ranges, and the final cost depends on the property’s site conditions, design, finishes, access and approval requirements. Additional work such as excavation, retaining walls, demolition, utility connections or other site-specific requirements can increase the overall project cost.
A granny flat in St Marys typically takes around 1–3 weeks for design and approval, followed by approximately 3–4 months for construction in straightforward cases. The timeframe can vary depending on the design, approval pathway, site access and conditions. Projects requiring a Development Application (DA), custom engineering, significant site work, service upgrades or additional approvals may take longer, so the expected timeframe should be confirmed after the property has been assessed.
Granny flats in St Marys can be approved through a Complying Development Certificate (CDC) instead of a traditional development application, provided the property and proposal meet the NSW Housing SEPP standards. For the standard CDC pathway, the lot generally needs to be at least 450m² unless the secondary dwelling is located entirely within an existing dwelling. If the proposal does not meet the complying development requirements, a development application or another approval pathway may be needed.
St Marys granny flat designs include 1-, 2- and 3-bedroom floor plans, with options for different layouts, facades and features. The most suitable design depends on the available space, how you plan to use the granny flat and the planning requirements that apply to the property. Designs can be adapted to suit site-specific factors such as access, privacy, outdoor space, setbacks and the position of the existing home. The final layout must meet the relevant planning and building requirements for the St Marys property.
Yes. Granny Flats X offers customisable 1-, 2- and 3-bedroom designs, so the layout and finishes can be adjusted to suit the property and how you plan to use the space. The final footprint still needs to fit the site and approval requirements. A simple, efficient plan is often the best starting point when the backyard has existing structures or limited access.
