Surface Development Guideline 1

Introduction

Guideline 1 applies to properties undermined by shallow historical mine workings with a high or moderate risk of a pothole forming on the surface based on the geometry of the workings, thickness of the coal seam, and the nature of the overlying strata.

Allowable residential construction

Guideline 1 allows for up to two residential buildings that conform with the following description:

Option 1:

A single or two storey residential development limited to a maximum length of 24 metres and maximum footprint size of 400m² designed in accordance with the following requirements:

• Maximum height of foundation brickwork of 1m. All masonry is to be articulated in accordance with the current edition of Australian Standards AS4773.
• Infill slabs on ground are only permitted in garages with a maximum area of 50m² and are not to be used to support any walls.
• Reinforced concrete perimeter strip footings that comply with the following table, excluding sites that would otherwise be classified as class H2, class E, or class P (unless classified as class P due to mine subsidence).

Note: All longitudinal reinforcement is to be placed inside steel ties.

Option 2:

A single storey brick veneer or two storey timber or steel framed residence with lightweight cladding designed in accordance with the following conditions / constraints:

• Designed by engineering principles by a qualified structural engineer to accommodate a pothole of maximum diameter of 5m forming at any point underneath the building structure. The structure should be capable of both spanning and cantilevering over a 5m diameter depression.
• Design drawings shall be submitted to Subsidence Advisory for acceptance prior to construction with the following written certification from a qualified structural engineer that the improvements will remain:

Safe, readily repairable, and any damage from mine subsidence shall be limited to slight damage in accordance with AS2870 (Damage Classification), in the event of a 5m diameter pothole forming at any point under the building.

If applicable, improvements should also be designed to accommodate any predicted ground movements that may be associated with founding on reactive clay soils.

• Infill slabs on ground only permitted in garages and of maximum area of 50m² and not used to support any walls.
• Certification by a qualified structural engineer upon completion that the improvements have been constructed in accordance with the approved design drawings.
• Masonry is to be articulated in accordance with the current edition of Australian Standard AS4773.

In both Option 1 and Option 2, concrete is not to be placed in the footing excavations before a Subsidence Advisory officer has inspected the reinforcement steel in position. 48 hours’ notice is required to book an inspection with Subsidence Advisory. Details on this process are included in the application determination letter.

Prohibited works 

The following works are not permitted under this Guideline:

• Basements (including partial basements where one or more walls acts as a retaining wall)
• Suspended, drop edge or stepped slabs
• Masonry internal walls
• Raft slab foundations – infill slabs not structurally connected to the building are permitted in garages only.

Commercial buildings or applications for more than two separate residential buildings require assessment by Subsidence Advisory in accordance with its Merit Assessment Policy.

Other allowable additions and improvements

The following detached structures on Guideline 1 affected properties will require certification from a qualified structural engineer that they will remain serviceable under a 5m diameter pothole:

• Concrete swimming pools
• Cabanas 
• Retaining walls greater than 1m high
• Detached garages (must be non-habitable space and limited to a maximum footprint of 50m²)

Granny flats (i.e., single storey lightweight structure with maximum internal floorspace of 60m² and sheet metal roof) shall be designed in accordance with the requirements as outlined in Option 1 or Option 2 of this page.

Exempt developments

In addition to the above, Subsidence Advisory has exempted some minor construction works from requiring approval under s22 of the NSW Coal Mine Subsidence Compensation Act 2017.

For a full list of exempt developments, please refer to Subsidence Advisory’s Exempt Development list.

Proposed developments that do not comply with the applicable Guideline

Applications for proposed improvements that do not comply with the Guideline assigned to the property may be assessed under Subsidence Advisory’s Merit Assessment Policy. 

Who can assess whether development complies with Guideline 1

Applications for proposed development on this Guideline must be assessed by Subsidence Advisory.

Disclaimer

Please note that Subsidence Advisory’s Surface Development Guidelines are subject to change.