Miscalculating Population and Housing Yield from Strategic Developments

Yesterday I read not a bad new book called Connected Cities from a journalist interested in planning, suggesting a ‘string of pearls’ approach to building new settlements .  Great, much like long suggested on this blog and in Peter Hall and Colin Wards Book Sociable Cities.

However it was difficult to take the book seriously as it didn’t make two essential calculations.

The first was how spare spare capacity there was on the railway and how it might be possible to boost the capacity to support the level of population proposed with the necessary modal split.

The second was it assumed that all roads were polylines consuming no space, and did not include land take for schools etc., SUDS, surface water storage and other non sellable land.  As a result it overestimates the number of dwellings at the proposed density by around 100%.

Interestingly Mr Ebenezor Howard and friends in their Garden Cities Challenge response did do a correct such calculation of their’land budget’  and modal split here.

The error is fairly common.  Quite often local plans include housing yield circulations for strategic sites based on gross to net ratios applicable to much smaller sites (30-40%), by omitting the huge land takes for secondary schools, connector and arterial roads etc.  when the governments own Harmen review (local plan delivery group) in 2012 stated:

One error that has a very large impact on the outcome of viability testing is
overlooking the distinction between the gross site area and the net developable area (ie. the revenue-earning proportion of the site that is developed with housing).
The net area can account for less than half of the site to be acquired (that is, the size of the site with planning permission) once you take into account on-site requirements such as formal and informal open space, sustainable urban drainage systems, community facilities and strategic on site infrastructure etc. On larger sites, sometimes the net area can be as little as 30%.

Lets be clear on terminology here.  By gross-new ratio  i’m referring to the proportion that is taken from a site to reveal sellable area (including retail and employment) which is then divided into sellable land uses of different types.  Internationally this is known as the ‘exaction rate‘  A term I hope we would use in the UK as it is precise and clear.  This varies by climate and geography.  You need less open space in desert climates but ironically much more land for surface water storage (flash flooding).   The figure is around 35%-60% internationally.  1 minus exaction rate in net developable sellable.  55% is a reasonable estimate given UK road (Design for Streets) and community services standards in my experience but will vary site by site i n terms of how much land is usable in any scheme (including flood plain.  This assumes semi natural greenspace which forms the borders of settlements are outside the ‘green line’ community area boundary for calculation purposes even if they are within any masterplanning ‘red line’ .  Care needs to be taken with screened out ‘undevelopable’ areas such as flood plain which are proposed to be zoned within any masterplan – such as for public open space.  Then , unless it forms an outside of community semi natural greenspace function should be included within the ‘green line’.

With much hard work on optimising layouts you can reduce the road take by several % but it is unwise to assume this at the outset it just pushes up the price of land.  Take a conservative assumption and then later optimise layouts and use the uplift to increase affordable housing.   Ensure the settlement wide population design size  and infrastructure load assessments include a 10% or so headroom allowance to encourage such optimised neighbourhood area masterplanning., but allow this increase by design review not as an ‘as of right’ zoning.

Similarly in the future new technologies such as self driving and parking cars may reduce the need for wider streets on main roads or two way streets on local access roads; however it is better to treat this as a bonus towards space towards people and streets and open spaces.  In one scheme recently I used saved space to enable cycle facilities to Dutch Crow design guide standards. The housing should be zoned in any event at an optimal density in terms of natural light and access to services and transit.

The only way to be sure of the figures on your land budget in any strategic planning areas is to do a ‘test fit’ using your local design guide and infrastructure standards of a reasonably representative strategic growth location or two, and even in such a test fit you only need to design one neighbourhood down to local access road standard to be able to calculate and apply that figure across all neighbourhoods designed down to local collector road level. .  How many areas do this if any?  I also note some local design guides only work for schemes of up to several hundred dwellings (Central Beds is good example -good design guide but only for small sites) in contrast to those for Essex and Kent for example which are designed for sites at all sizes.  Of course we also have the problem that many counties still run pre design for streets roads standards (such as Herts and Gloucestershire), and all of them provide too little space for cycling.  This I would not worry too much about as if you design a masterplan to CROW standards with a 250m Dutch cycling grid of dedicated routes you shift the modal split so much you need a much lower land take of major collector/link roads and minor collector /feeder roads and the land budget balances out.

If any areas wants such an assessment done i’m available.  If we spend such a large amount of money running spreadsheets on viability on proposed allocations at least we can spend a little on getting the pends and cad out for them as well.

Doing this transforms the strategic planning exercise into much more of a positive design and landscape led process where transport and pother problems that might have held back such sites present themselves with creative solutions.  It also moves beyond the ‘fuzzy felt planning’ trap of big blobs envisaged as concrete and tarmac rather than active placemaking which can often enhnce green infrastructure and natural capital on a site. The best way to derisk is to design.

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