Could a variation to the territory plan bring about a greener future?

However, Draft Variation 369 (Living infrastructure in residential zones) is an exception that is well worth a read at any time of day or night, and the ACT planning authorities should be congratulated for producing it. Its overall objective is to achieve a 30 per cent tree canopy cover, and 30 per cent in permeable ground surfaces in urban areas by 2045. More shade, and the cooling effect of transpiration from vegetation, is expected to reduce temperatures in built-up areas. Improved surface absorption of rain will reduce the amount of runoff entering stormwater drains, as well as recharging underground aquifers.

In response to growing concerns by many Canberrans about the general loss of tree cover across the city in recent years, the government proposes to limit the footprint of new residential dwellings (the "site coverage") to a smaller percentage of their block. The remainder of the block would constitute "private open space", a percentage of which would be required dedicated to "planting space" for trees and other vegetation. Unlike the current situation, the definition of private open space would exclude impermeable items such as swimming pools, terraces, pergolas, etc.

A block of up to 800 sq m would require 60 per cent to be left as private open space, with half of that space to be permeable planting area suitable for deep-rooted trees. A further requirement would be that the planting area contains at least one small, and one large "canopy tree". Similar rules are envisaged for larger, medium and compact block sizes. Analogous provisions would also be mandated for multi-unit housing developments of varying dimensions.

A number of residents' associations have welcomed the government's initiative. However, considerable scepticism remains about any government's long-term ability to apply and enforce the new rules. There is also some foreboding that the construction industry will actively oppose a more stringent approach to site coverage, and that the rules may be whittled away bit by bit through political influence in the future.

In particular, it is not clear what enforcement mechanisms would be relied on by future ACT governments to ensure that the new rules are effective.

Concern has been expressed at public meetings of residents that even existing planning rules are not strictly enforced, or result in houses which formally meet the rules while being clearly inconsistent with the spirit of the rules. Many readers will be aware of large houses that cover most of the block, with the remainder concreted or paved so that little or no permeable surface or vegetation is left. Another example is the approval of redevelopments with underground garages, but where owners later convert their garage into living space contrary to current planning rules. No systemic follow-up monitoring and enforcement appears to be pursued by the ACT planning authorities.

In themselves, the proposed new rules do afford some scope for evasion and deception in the absence of continued monitoring and enforcement. Trees can be removed after initial planting, and planting space concreted over. Tree canopies may be severely pruned because they are too close to power lines, or interfere with guttering on roofs; a likely outcome if the trees have been planted in an inappropriate location.

Active monitoring and enforcement of the proposed new rules after construction has been completed and approved, would require the dedication of considerable government resources, and would necessarily be intrusive. And the resource cost would increase over time, as more of Canberra's older homes are demolished or renovated. Nor would the new rules alone require or encourage owners of older, unaltered dwellings to plant deep-rooted trees or other vegetation, or to remove impermeable surfaces on their property.

An alternative approach to maintaining tree planting and surface permeability across Canberra would be to offer financial incentives in the form of reduced rates. Properties that meet the new rules on planting area, for example, could qualify for a 5 per cent reduction in annual rates, whether they contain a new dwelling or a pre-existing one. A further 10 per cent reduction in rates could be provided to owners who maintain suitable canopy trees.

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An aspect not covered by the proposed new rules is the potential benefit of pollarding or coppicing, where large tree canopies are not practicable. It is feasible to plant large, deep-rooted deciduous trees such as sycamores, even in limited space on the northern or western side of a building. Once the trees reach a certain height they can be pollarded or coppiced, with multiple branches continuing to grow and shade the external walls of a building. Newer suburbs with smaller blocks would thus be able to take advantage of similar rate reductions offered to property owners in more established areas.

A rate-reduction scheme commands a respectable position in economic theory. Property owners who maintain large canopy trees and permeable surfaces generate public benefits (positive social externalities) for other residents, particularly in summer. Provided that the social (public and private) benefits can be shown to exceed the social costs in a rigorous cost-benefit analysis, a rate-reduction scheme would be justified.

The budgetary effect on government finances would also need to be considered. But outlays could be contained within reasonable limits by relying on self-reporting by residents regarding their maintenance of trees and permeable surfaces, with inexpensive random audits using drones. Alternatively, occupancy permits for new houses could be issued subject to compliance with the rules. Restrictive covenants attached to properties as part of an occupancy approval would ensure that future owners would hesitate to buy a non-compliant property.

Comments on the ACT government's proposed changes can be made until Friday, May 8, at planning.act.gov.au/planning-our-city/territory_plan/draft_variations_to_the_territory_plan

There is no point losing sleep about urban heat island effects. An innovative, market-based approach to adaptive planning can let everyone chill out in a greener environment.