Look at planning laws around Australia and you have to wonder what some people are thinking.
According to the ABS, in 2011-12, the average Australian man (18 years and over) was 175.6 centimetres tall. The average Australian woman was 161.8 centimetres tall. There is no statistical deviation depending on which state or council area you live in. However, looking at planning codes around the country, you’d be excused for thinking otherwise.
An ad-hoc desktop straw poll of floor-to-floor and overall height allowances in different council jurisdictions revealed significant cause for confusion on what appropriate standards should apply. Policies on how height is assessed vary significantly from council to council and state to state. A few examples:
- In the Arden Macaulay Structure Plan 2012 Melbourne, a storey is defined as having a 3.5 metre floor-to-floor height for residential uses and four metres for non-residential uses
- According to the New Residential Zones and Mandatory Maximum Heights Glen Eira Council 2014, 13.5 metres is the maximum allowable height as specified in the Schedule (effective floor height of 3.375 metres)
- The Northbourne Precinct Master Plan (ACT) 2014 specifies that for 25-metre high buildings, the number of storeys is determined on the basis of nominally three-metre floor to floor heights
- A 2013 Amendment to the Sydney Development Control Plan 2012 states that a floor to ceiling height of 2.7 metres achieves the floor to floor height of 3.1 metres, which is the minimum to allow for finishes and construction tolerance
- As noted in the Darebin Planning Committee Minutes Nov 2013, in strategic locations where there are no amenity impact issues to consider, it is accepted planning practice to allow variations to height controls subject to offsets provided by architectural excellence and high standard of environmental sustainability
- Why, from the planning perspective, a nine-storey building of 26.1 metres would be unacceptable when an eight-storey building of 32 metres is acceptable, is not apparent, as outlined in the State Administrative Tribunal of Grove & Ors v. City of Stirling  WASAT 136
- At that same tribunal, it was noted that ‘A number of anomalies (exist) in the height in storeys control for properties in the suburbs of Darlinghurst, Elizabeth Bay, Paddington, Potts Point, Rushcutters Bay, Surry Hills and Woolloomooloo.’
Perhaps the master planners in Canberra really thought their residents are on average half a metre shorter in stature than those in Melbourne? It is a mystery that in a modern federation with national building standards, such fundamental discrepancies can still exist in planning.
Planning submission design often finds developers and architects grappling with yield objectives on one hand and council planning height controls on the other. The ideal for designers is that height controls are framed so as to allow designers to work through alternative structural systems after planning applications are completed.
Whilst architects consider structure in principle at planning stage, a structural system is optimally not fixed until a design development/cost-planning process is undertaken. As design for a project develops, performance criteria for structure is balanced against acoustic and fire rating performance standards, to achieve cost effectiveness, compliance and good amenity design outcomes.
If controls are set in terms of number of storeys rather than proscriptive overall height in metres, the design development process can be creative and interactive as architects work with structural and other consultants to test various system options and to find the most efficient solution. This ideal is far from reality in many areas of Australia. Even though the principle of ‘generally in accordance’ is supposed to allow some design flexibility, over time with precedents set by appeal interpretations in relation to height, I would argue that confusion on this issue is still widespread.
During a recent presentation of an engineered timber solution to a design and construct builder, the question arose of potential impact on an existing planning approval. This is a common concern for builders who have not worked in timber.
The uncertainties of planning lead to defensive design attitudes by developers and builders. The widespread inconsistency of height controls across council areas supports a perception of risk that has downstream impacts on perceived design flexibility when it comes to alternative structural solutions. Even though light timber systems can effectively save on floor depth by allowing services access through the structure, there is nevertheless a market perception that a timber floor structure is deeper than concrete. The consequence is that design teams may not explore timber unless there are no possible planning impacts.
Structural system choice may depend on design spans, load paths etc. but may also depend on a particular builder’s trade preferences. To the uninitiated, the variations may not seem that significant. However for the designer, the flexibility to accommodate say an extra 50 millimetres in floor-to-floor tolerance can make the world of difference. Without it, and when tolerances are compounded over a number of levels in a multi-storey project, it can negatively influence the structural design process.
A structural timber system will inevitably utilise layers of tested and approved products to achieve fire and acoustic performance. Just as designing with a concrete structure has to take into account fitting services under thickened beams, the overall depth of a load bearing timber structure will depend on accommodating the sound attenuation and fire layers required.
A system recently tested by CSIRO for Australand at “THE GREEN” Parkville Victoria featured:
- Two layers of an 18-millimetre thick Promat SYSTEMFLOOR
- A 350-millimetre TECBEAM floor joist
- CSR R2.0 SOUNDSCREEN batts
- A 20-millimetre PROMATEKT 100 fire barrier
- Rondo suspended ceiling system
- CSR R2.0 SOUNDSCREEN batts
- A 13-millimetre fire rated plasterboard ceiling
- Nominal 190-millimetre vertical depth ceiling cavity
- Achieved airborne performance Rw+Ctr 52 (equivalent to DnT,w+Ctr 47)
It is the sum of these layers; the overall composite system – flooring, joists, fire rated soffit, insulation and so on that ultimately determines performance. For a premium system and over multiple layers, overall building height may sometimes have to be adjusted to accommodate the structural design approach.
Clearly if planning height controls so narrowly proscribe floor to floor or overall building height as to discourage investigation of the full range of structural solutions, this may impact consideration of timber as the viable alternative to concrete.
This consequential impact of height controls is particularly relevant to low to medium-rise sites – up to five or six storeys, where timber load bearing structure is now recognised by leading developers as an economic option that can save developers and builders both time and money.
This is problematic for many reasons, but particularly so as a constraint on use of timber, would contradict the intent of sustainable design policies in many council areas. Timber is widely recognised as a more sustainable building choice. The arguments for timber construction are compelling. In New Zealand the Ministry of Agriculture and Forestry recently conducted a research project to compare the sustainability of timber with steel and concrete. Timber won by a sizeable margin. Concrete emits nearly its own weight in carbon dioxide as it’s produced, while wood sucks carbon out of the atmosphere as it grows.
We must question whether sustainable design in timber, with performance criteria targets at or above BCA, should be compromised because designers are effectively constrained within inconsistent and practically inflexible local planning height controls. Clarity that the flexibility exists to utilise timber systems in order to deliver good environmental outcomes, as well as cost efficiency and high performance standards is now urgently needed.
Likewise, planning controls that neglect to explicitly allow provisional flexibility for structural system choices should not disadvantage the timber industry. It is time for all councils to remove the uncertainty and explicitly clarify, or if necessary amend height controls to best practice. This was demonstrated in the recent Residential Flat Design Code prepared by the NSW Government, which adds the qualification to all height standards to “allow for construction tolerance.”