Sheds, decks and outbuildings: do they need a BAL?

New houses get most of the attention, but a surprising share of the assessments we do are for sheds, decks, carports and secondary dwellings. The rules are simpler than for a house, but they catch people out, usually after the shed has been ordered.

The 6-metre rule

In a high bushfire risk area, any outbuilding within 6 metres of a dwelling must be built to the same BAL rating as the dwelling. The logic is straightforward: a shed that catches fire 4 metres from your house is a threat to the house, so it has to meet the same standard.

That means if you are putting up a shed, carport or deck near the house and you do not already have a BAL rating for the site, you need one. The CFS and Adelaide Hills Council both note that these structures often require a private BAL assessment as part of building approval.

Beyond 6 metres

A detached shed more than 6 metres from the house generally does not need to match the dwelling's rating, though council may still have requirements depending on the overlay and the size of the structure. Sheds over 500 square metres fall under commercial provisions. Check with your council's duty planner before assuming a large shed is exempt.

Decks and verandahs

Decks attached to the house are part of the house for BAL purposes. At BAL-29 and above, decking within 300mm of the building typically has to be non-combustible or a bushfire-resisting timber, and the subfloor space has to be enclosed or screened. This is one of the most common items that gets missed in renovation plans, because the deck feels like landscaping rather than construction.

Granny flats and secondary dwellings

A habitable secondary dwelling is treated as a dwelling. In the High bushfire hazard overlay that means a mandatory CFS referral, the same as a new house, and the CFS sets the BAL as part of that process. We would not charge you for a private report in that case; it would just duplicate the CFS one. If the flat is in the Medium risk overlay, the Code deems the land BAL-12.5 for planning purposes; unless it sits within 100 metres of a High risk area, in which case MBS 008 requires a site assessment. Check with council which applies.

What to do before ordering

1. Check whether the property is in the High or Medium bushfire overlay.
2. Measure the distance from the proposed structure to the house.
3. If it is within 6 metres in a High overlay area and you do not have a current BAL rating, get one before the shed is specified. The rating determines cladding, roofing and door requirements, and it is much cheaper to order the right kit than to modify one on site.