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A Proposed residential density control:

Why we need an alternative model

The usual planning controls in residential areas in New Zealand severely restrict design freedom and all to often result in ugly and inefficient buildings. In areas with high land values and small sections, recession planes and height limits encourage low spreading buildings with hipped roofs and a narrow strip of barely usable garden around them, too narrow to enjoy much sunlight. There is rarely room for trees in the gardens. The deep floor plans result in limited daylight to the interior. Where two storeys are required there is insufficient room for attractively shaped rooflines, and houses are generally built to the allowable envelope. There is usually little scope to compose building frontages to be attractive from the street, and first floors tucked into rooflines tend to have poorly placed windows. Areas with most of their buildings fitted to recession planes tend to have generally unattractive buildings, especially when compared to older developments of similar density built when such rules did not apply.

Where higher densities are required under these rules it is generally only possible when multiple units are developed together, such as in terraced housing and semi-detached townhouses.

Which one do you want to live in?

Overview of the proposed model

This model will need to be calibrated to the different living zones, but has the advantage that it is easily tuned to a wide range of densities. It would have three primary components as follows:

Volume Control

By controlling building volume directly it should prove to be a more predictable tool for controlling development density, while allowing for more individual design freedom, and encouraging better and more attractive design. While there would be no height limit, the built volume control would result in taller buildings being narrower, allowing more light around them. There would be no incentive to build very tall houses since such buildings would necessarily have very small floor plates, and be proportionately expensive. A few slender tower houses though would add interest to the streetscape.

Site Coverage

Site coverage restrictions would apply much as they do now, though developers should be discouraged from imposing minimum site coverages as part of their development covenants. With taller houses it may be desirable to slightly reduce allowable site coverages, especially where two or more stories are encouraged, so as to allow for more green space and trees, while still maintaining overall density.

Two Tier Setback Rules

Setback rules would be used in a similar manner to that used now, except that front, rear and side setbacks would be calibrated for sunlight access, generally around rather than over adjacent buildings. A rear setback line would help ensure light to back gardens. In very high density areas the side setbacks may be set to zero, or perhaps only a driveway width setback on the northernmost side. In a back section or a section oriented with its long axis parallel to the street the back setback lines would be re-oriented to give a squarer garden side, generally closer to the north end of the section. There may be scope to relax the back overall setback line for a courtyard house.

In addition to the overall setbacks beyond which a building would not be able to project, there would also be an attenuation setback line, set a distance inside the overall setback line. Between these two lines would be projecting portions of the house, but only a reduced portion of its bulk. A plain cubic building would need to fit entirely within the attenuation setback lines, though a projecting wing could occupy this space. This rule would encourage shaped buildings, and remove any temptation to simply build to the allowable envelope. Simple cubic massing or long planar sides would be allowed, but there would only be an incentive to do so as part of the architectural intention, not as a simple economy.

Maybe this will make it clear...

Detailed principles

Volume Control

The allowable building volume (V) would be determined based on the site area (A), determined as a proportion (pv) of the average cube volume of the site (CV). This could be calculated using the following formula:

    CV =  A x √A

This can work independently of the geometry of the site and is easy to calculate. It will also give quite consistent results over a range of site sizes. The allowable volume would then be given by:

    V = pv x CV

In this case pv might be 0.25 in a high density area, down to as low as 0.075 in a low density outer suburb or green belt, with pv being chosen for each living zone to give on average a similar maximum density to that dictated by the current rules for that zone.

An alternative approach would be to substitute a set height (H) instead of √A, say 80% of the height limit set out in the current or proposed plan. In this case the allowable volume could be dictated by the allowable percentage site coverage (pc) and the site area, and the allowable volume would be given by:

    V = pc x A x H

Here H might be 12m for a high density area, down to say 5m for a low density area, the rest of the difference in density coming from the different site coverage set out for each zone. H would not be a height limit as such, but rather a factor for calculating the allowable volume (V).

Site Coverage

Site coverage would be administered much as it is now, with the allowable site coverage expressed as a percentage (pc) of the site area.

Two Tier Setback Rules

Between the overall setback line and the attenuation setback line, the average cross-section area of all sections cut between the two lines should be no more than a set percentage of the greatest cross sectional area of the house parallel to that setback line. 35% should be quite sufficient to allow for projecting wings, porches, bays and other projections. By this reasoning if a building were to have no projections at all its outside wall would then be allowed to stand 35% of the way between the attenuation setback and overall setback lines.

In a higher density area the front overall setback line might be 1.5m in from the street boundary, with the attenuation setback line 1m further back. In a low density area the overall setback line might be 5m from the street boundary, with the attenuation setback line 2m further back. In a higher density area the back overall setback line (or garden overall setback line) might be as little as 2m from the back boundary, though the back attenuation setback may be say 4m inside that line to allow for a projecting wing of the house sheltering the garden. In a low density area the back overall setback line could be up to 5m from the boundary, with an attenuation setback line another 6m back. On the south side of a street these setbacks might be reversed to allow for a garden to the front instead.

In a very high density area the side overall and attenuation setback lines may both be set to 0, allowing for a terrace type density with individual houses. At slightly lower densities the northernmost overall side setback may be set to 2.5m with an attenuation setback of 1m to allow for a driveway, while the southernmost side boundary might have an overall side setback of 0, and an attenuation setback of 0.6m. In this case there might be no side fences alongside the houses, and with a guaranteed separation of 2.5m, dispensation could be made for service room windows overlooking the neighbouring driveway, provided that the windows were glazed and configured so as not to have a view out. In low density areas the side overall setback could be say 3m with an attenuation setback of 1.5m on the northernmost side, with an overall setback of 1m and an attenuation setback of 1m on the southernmost side boundary.

In any case the setbacks should be fairly consistent along any given street to ensure the houses along a street get as much sunlight as practicable, and no-one is overly disadvantaged. It would also be desirable to exempt free-standing ancillary buildings such as garages, workshops and studios from the setback lines, though not from the site coverage or volume control rules. Ancillary buildings in breach of the setback rules could be subjected to recession plane rules, at least on south boundaries.

James Carr - October 2011