Granny flats, secondary dwellings on the same lot as a main house, are one of the most popular property improvement projects in the Blue Mountains. State Environmental Planning Policy (Housing) 2021 simplified granny flat approval in NSW, enabling most complying granny flats to be approved as complying development without a full DA.
But while the granny flat itself may qualify for fast-track approval, the site preparation works, particularly retaining walls on Blue Mountains steep lots, often don’t. Understanding how the retaining wall component of a granny flat project is approved and what it costs is essential to realistic project planning.
The Challenge: Steep Lots and Granny Flat Footprints
The Blue Mountains has excellent development potential for granny flats, lots are typically large enough to accommodate a secondary dwelling with adequate setbacks. The challenge is the terrain.
On a typical Blue Mountains steep block:
- The main house sits on one level pad
- The garden falls away steeply below or to the side
- A granny flat at garden or driveway level requires either a large cut into the slope or significant fill and retaining below the flat
In either case, retaining walls are central to the site preparation works.
Common granny flat site configurations in the Blue Mountains:
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Below the main house: Granny flat carved into the slope below and to the side of the main house. Requires a cut and retaining wall at the back of the flat, plus potentially retaining on one or both sides.
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Terrace level below the main house: Granny flat on a new level terrace created by cut-and-fill below the existing house. Requires retaining walls around the full terrace perimeter.
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Extended garden level: Granny flat at the bottom of a slope, with the slope above retained by a wall to create a flat area between the new flat and the existing slope.
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Beside the driveway: Granny flat in the area beside the existing driveway, often requiring retaining on the uphill side.
How Retaining Walls Are Approved as Part of a Granny Flat
The critical issue: the granny flat itself may qualify as complying development under the Housing SEPP, but this doesn’t automatically extend to the retaining walls needed to prepare the site.
Retaining walls under 1.0m that meet the standard exempt development conditions (not in HCA, not near drainage line) can typically proceed as exempt development, even if the granny flat is being built under a CDC.
Retaining walls over 1.0m generally require a DA from BMCC, regardless of the granny flat’s approval pathway. This is a common surprise for homeowners who expect the granny flat CDC to cover everything.
Two-pathway projects: Many granny flat projects in the Blue Mountains end up with a dual approval process:
- Granny flat building work: CDC through an accredited certifier
- Retaining walls over 1.0m: separate DA through BMCC
This dual pathway adds time and cost but is the correct process. The DA for retaining walls should ideally be lodged and approved before or alongside the CDC for the granny flat, to avoid delays in construction.
See [/guides/bmcc-council-approval-retaining-walls/] for the full approval guide.
Engineering Requirements for Granny Flat Retaining Walls
Retaining walls built as part of granny flat site preparation face several engineering requirements beyond standard residential walls:
Wall Below the Main House
If retaining walls are built in proximity to the main house footings, the wall design must account for the load from the house (surcharge). The engineer must assess the footing conditions of the main house and design the retaining wall to avoid undermining the main house foundation.
See [/guides/retaining-wall-below-house-foundation/] for the setback rules and structural risks.
Wall Beside the Granny Flat
The granny flat will typically have footings that bear adjacent to or near the retaining wall. The retaining wall footings and the granny flat footings must be designed together, conflicts between the two footing systems can cause one to undermine the other.
The structural engineer (or separate geotechnical engineer) should review both footing designs together before construction starts.
Fill Below the Granny Flat
If the granny flat floor slab will sit on fill material (placed during the site preparation earthworks), the fill must be compacted to a specification that will support the slab without settlement. Uncompacted fill under a slab causes differential settlement, cracking, and structural damage over time.
A geotechnical engineer should specify the fill material and compaction standard. This is particularly important in the Blue Mountains where some fill material (clay-rich soil from the cut) is not suitable as structural fill without treatment or replacement with granular material.
Drainage Integration
The granny flat’s stormwater system and the retaining wall drainage system must be designed together. Common issues on steep Blue Mountains sites:
- Drainage from behind the retaining wall cannot discharge toward the granny flat (flooding risk)
- The granny flat’s roof water must be directed to an approved stormwater point, not toward the retaining wall
- A subsoil drainage system behind the retaining walls must connect to the property’s stormwater system with capacity to handle both sources
See [/guides/retaining-wall-drainage-design/] for the drainage design guide, and ensure your builder/certifier integrates the retaining wall drainage into the overall stormwater plan for the granny flat.
Access During Construction
Building a granny flat on a steep Blue Mountains site almost always requires careful planning of construction access. Retaining walls are typically the first structural work done (you can’t build the flat until the site is prepared), which means:
- Large earthworks equipment (excavator) must access the site
- The driveway must be able to handle concrete deliveries for footings
- H-post delivery requires a vehicle that can reach the construction zone
- If existing landscaping or fencing blocks access, this must be removed and reinstated
Plan access requirements before starting construction documents, restricted access sites in the Blue Mountains add significant cost.
Timeline and Cost Impact
Retaining walls are often the critical path item in a granny flat project on a steep Blue Mountains site:
Timeline:
- Retaining wall DA preparation: 4-8 weeks
- BMCC DA assessment: 40-90 days (standard track)
- Construction of retaining walls: 2-6 weeks depending on complexity
- Site stabilisation and drainage: 1-2 weeks
- Granny flat construction can start after retaining wall and earthworks are complete
Total site preparation time before granny flat construction: 4-6 months is typical when a DA is required.
Typical costs for retaining wall component of a granny flat project:
| Item | Typical Cost |
|---|---|
| Geotechnical assessment | $1,500 to $3,500 |
| Engineering design (retaining walls) | $2,000 to $5,000 |
| DA preparation and BMCC fees | $3,000 to $8,000 |
| Retaining wall construction (15m × 1.5m high) | $17,000 to $28,000 |
| Earthworks (cut and fill for flat site) | $8,000 to $25,000 |
| Drainage system | Included in wall or $2,000 to $5,000 additional |
Total site preparation costs before the granny flat construction itself: $25,000 to $60,000 is common on steep Blue Mountains sites. This is a significant component of the overall project cost that granny flat builders often underquote or omit from initial estimates.
See [/guides/retaining-wall-cost-guide-blue-mountains-2026/] and [/guides/cut-fill-earthworks-blue-mountains/] for detailed cost breakdowns.
Slope Stabilisation as an Alternative
On very steep sites where conventional retaining walls would be very tall or very expensive, slope stabilisation approaches can provide a more economical solution. Engineered batters with vegetation, rock anchors, or geosynthetic reinforcement can sometimes replace or reduce the size of retaining walls needed for a granny flat site.
See [/services/slope-stabilisation/] for the options.
FAQ
Can a granny flat retaining wall be included in the CDC process? Only if the retaining wall is under 1.0m and meets all exempt development conditions. Walls over 1.0m require a separate DA from BMCC, even when the granny flat building work is approved via CDC. The dual-pathway process adds time but is the correct approach.
My granny flat builder says no council approval is needed for the retaining wall, is that right? Only if the wall is under 600mm and meets all exempt conditions. Walls over 1.0m definitely need a DA. Be cautious of granny flat builders who understate the approval requirements for site preparation works, the liability for unapproved works rests with you as the property owner.
How long will retaining wall approvals take for my granny flat project? A BMCC DA for retaining walls typically takes 40-90 days. Factor this into your overall project timeline. Starting the DA process as early as possible, before finalising the granny flat design, is strongly recommended.
Can the retaining wall and granny flat DA be lodged together? In some cases, yes, a single DA can cover both the granny flat and the associated site works including retaining walls. This can reduce approval time and fees compared to two separate DAs. Discuss with BMCC at the pre-DA stage.
Who designs the retaining walls for a granny flat project? The structural engineer working on the granny flat will often also design the retaining walls, or a specialist retaining wall engineer can be engaged separately. The key is that both the granny flat footings and the retaining wall footings are considered together.
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