Tall Buildings and Density Debate
Notes and reflections of an event organised by Urban Design London
Background

The population of London continues to increase:
 2011 – 8.17million
 2036 – 9.33 million
 85k increase per annum



Over 200 towers above 22 floors in height are proposed in London
80 percent of tower accommodation is residential
Latest construction technology now enables “super slim” buildings to be built to heights of 1000ft,
which are being introduced in New York.
Towers can be designed to be “iconic” or as part of cluster
On the whole the schemes are presented and considered in isolation; even in relation to adjoining
proposed development.
The London Density Matrix appears in the main, to be ignored. With the majority of
developments proceeding at a higher density than the appropriate range in the Matrix.



London density matrix





It is based on
o an assessment of the character of the local area as: suburban, urban or centre
o an assessment of Public Transport Accessibility Levels, using the PTAL method.
It was developed from a series of 50 design studies
Has been little studied or challenged in academia
The use of PTALs as one axis of the matrix as a proxy for transport costs may not be particularly
academically rigorous.
PTALs were not part of the original matrix developed from research. This used a 1-6 grading for
number of facilities within a short walk of the site. Measuring something totally different to PTALs
Describing Density
Descriptions include
 Population density
o Variants include child density; tenure is an issue.
 Dwellings per hectare
o Variants – habitable rooms per hectare
 Employment density
 Plot ratio (also known as floor : area ratio)
o A plot ratio of 1 that includes roads and public spaces, is quite dense
 Gross v Net
o As soon as you add roads and public realm density dilutes
o Residential density figures are normally based on net residential area, which includes integral
roads and ancillary open spaces
o Infill schemes come across as “higher density” than large schemes, as they don’t have the
open spaces which larger schemes do.
o An understanding of an appropriate carrying capacity or gross density for a neighbourhood
may be a useful planning concept.
 Mixed use presents problems for conventional descriptions of density
Descriptions of Public Transport Accessibility
PTAL Public Transport Accessibility Level
 Is based on the time taken to walk to public transport service access points such as bus stops
and stations.


It is publically available on www.webptals.org.uk
PTAL also reflects street connectivity – for example culs de sac vs grids
PTAL limitations - there are other factors which influence access that are not included within the
PTAL measure such as:
 Quality and capacity of pedestrian environment
 Ease of interchange
 Crowding and capacity
 Speed and utility of accessible services
There are some anomalies with PTAL: Tooley Street (next to London Bridge Station with the platform
6 – the UK’s busiest platform) has a PTAL rating of 2. (low)
PTAL makes assumptions on walking speed and time maximum time spent walking – using a walking
speed of 1.3 metres per second walking speed. Recent research has recorded free flow pedestrian
speed with an average of 1.25 m/s, and minimum and maximum values of 0.68 and 1.92 m/s.
CAPITAL
This is an estimate of journey time through the network including the walking journey made at either
end. Currently it is only available within TfL.
CAPITAL is a tool to tell us how far someone can get in a set time from a set place. It explains the
richness of longer distance travel opportunities
TfL is soon launching WEBCAT a online tool to allow people to generate combined PTAL and
CAPITAL scores for sites. This could be very useful for density policies and may blow the matrix and
its use of PTALs out of the water.
Discussion
Longer distance transport is being used as an important determiner of development – yet a good
proportion of movement is economically unproductive.
Affordability of transport is not factored in, or the mobility of different portions of the population such
as the disabled or elderly.
Public transport is important in supporting active travel. For many city dwellers the walking
associated with public transport is all the exercise they get.
There remains a logic in relating density to public transport accessibility – but local facilities and
services should also be considered, as in the original Matrix.
Energy use in transport and density – is there an optimum density that
minimises transport energy use?




Horizontal transport costs (cars, buses, trains etc) increase as density reduces. Low density
suburbs are car dependent, whereas in high density towns and cities, walking can meet most of
the demand for personal movement.
Vertical transport costs (lifts and escalators) increase as density increases.
Research shows that there is an optimum building height of around 7 stories where the vertical
and horizontal transport costs are minimised. 7 stories was the height of many 19th century
European cities. (equivalent to around 250 dwellings per hectare, plot ratio 5:1)
If energy use is no object then a dense core with its own transport system combined with sprawl
can materialise as has been seen in developments in the Gulf.
Problems of higher density
Life
Cultural tradition for high density living - is arguably absent in London. There is no “concierge
culture” or enough recognition of the importance and challenges of maintenance and management at
higher densities.
Questions of health and wellbeing
- Sleep – impact of noise and light pollution
- Air quality
- Sunlight / Vitamin D deficiency
- Exercise
o Musculoskeletal disorders, arthritis etc
o Obesity
o Diabetes
o Blindness (Diabetes a significant cause)
- Mental health, stress and anxiety
o Impact of presence or absence of substantial amounts of greenspace
o Ability to see greenspace from one’s home.
- Social contact – loneliness
- Social mix – mixed tenure, mixed income, mixed age and stage of life, mixed ability.
- Personal Security
- Sense of community
o Pro-social vs Anti-Social behaviour
o Anonymity
- Friendship – at a local level
o Social care – by friends or the state
- Fun and enjoyment
Financing
Higher up-front costs
There are substantially greater up-front cost and hence financial risk for the developer in a high
density development before they can start selling units.
Underground parking costs around £18k per bay to construct – and for the developer is an upfront
costs that increase their exposure to changes in market.
Major housebuilders are used to building at lower densities and may typically budget only £50 per plot
for layout design – high density development represents a step change.
Location
Some high density developments are located along transport corridors
 where in the case of roads there is likely to be a concentration of air pollution
 where the night time noise levels are in excess of 60dB(A) Lnight
 where there may be limited local access to services and amenities
Under the old Planning Policy Guidance 24, these locations would have fallen in Noise Exposure
Category C – normally refused, unless no alternatives exist - conditions required, as it would be
challenging to provide sufficient noise insulation to reduce interior noise to a level that meets WHO
guidance on sleep disturbance.
Noise insulation has moved on since that time: it is possible to create buildings which provide quiet
conditions – but this means triple glazing and the inability to open windows means that air
conditioning, with an air supply that is well away from the exhaust fumes of the street,
Eg 6mm exterior glass, 18mm air gap, 12mm laminated glass, 100-200mm air gap, 4mm glass
Serious questions arise as to whether the planning system is providing healthy conditions for which
people can live.
Is the link between distance travel and density is getting too much attention, forgetting other important
local transport, health, environment, residential support functions etc considerations.
Scale
City scale
Is there an awareness of the evolving structure of the city? For example high density apartments on
the Isle of Dogs being occupied by high-earning city workers; whereas high density flats further out
being occupied by poorer people, and families? The different groups have different demands for
community/social infrastructure. Mixed communities policy may be failing at a city scale with
segregation occurring by income and wealth.
Is it realistic to provide extra accommodation without developing on new land?
Neighbourhood scale…
 Is sufficient consideration given to whether too many people are being concentrated in an area.
This issue needs considering at the neighbourhood, or wider level – not just sites.
 Do we properly consider how developing one site to a high density might effect development
opportunities elsewhere in the area? Is it right if one building blights the chances for another?
Site level - Sunlight, Daylight and Solar Gain, urban heat islands.
The creation of a new tall building can rob adjoining buildings of light and heat gain.
 Solar gain, shading, energy load
 Light at ground floor level and health implications
 Local heat islands – which relate to air mixing, and air conditioning.
Masterplanning needs to cover solar Masterplanning



Most of the discussion centres on the profile of the building, and most of the visualisations present
the new blocks as seen from the roof-tops of adjoining buildings.
Tower designers don’t illustrate their buildings in context – proposed adjoining building are
generally ignored.
They can also get very involved in discussing the overall shape, composition and balance of an
individual tall building, but they are hardly ever seen from ground to top by anyone once built.
People see the bottom part from close by and the top part form a distance. These two elements
are arguably more important that integrity of composition of the building as a single thing.
Street Scale – open planning…
 New areas of London appears to be a different scale than the older city, with – wider avenues
 Width of roads is important to people. In some areas with wide roads, though there are people
there they can appear lightly occupied.
Public realm and Streetscene
Public realm in front of high density and tall buildings is frequently poor. Because of the extra
population it justifies extra expenditure.
The attractiveness and liveability of the public realm is important in encouraging people to walk to the
public transport
Public spaces
When building at higher density, many things may be required of the available public spaces – many
different activities at different times of the day.
Building design
 The lower floors of towers are hugely important to their impact on a neighbourhood. This is a part
of the design that is frequently overlooked. It is this aspect of the design: the pedestrian view, that
is most important.

Many Tall buildings fail at the ground floor level, insufficient attention is paid to how a building will
appear to pedestrians.

Cladding systems have an uncertain life expectancy. There have been instances of buildings
shedding cladding and window glass. The UK’s oldest clad building – Peter Jones – has had its
cladding replaced twice in its lifetime. The need to provide for replacement cladding does not fit
in well with current mortgage models. How will todays tall residential towers be maintained and
reclad when needed?

Downdrafts from tall buildings can lead to extreme winds at ground level. Nobody seems that
sure about how good the modelling is, or whether this aspect is being properly and consistently
assessesed. One comment made that there will be wind, live with it.

Car parking – a stress area that is not resolved in many designs. The problem that with undercroft or underground parking, developers need to find money upfront, placing themselves at
financial risk. Advice is given in “Space to Park”

Waste management – conventional EuroBins 1100 Eurobins require massive storage space and
frequent lorry movements, that increase risks to other road users. There was a feeling that we
are underestimating the number of rubbish trucks that will have to visit a tall residential tower a
week. Could be very intrusive. Other options for waste movement away form site under ground
might work, but will they in areas with many tall buildings?
General Discussion
Planning system
The appeals system appears to permit developments that are poorly designed.
Economics and society
London is not just so many million homes, so many thousand miles of roads and streets, so many
parks. London is the whole thing put together. Someone, somewhere needs to understand what you
get when you stick all the bits together and what happens when you don’t.
Much of the discussion focuses on housing numbers, development economics, buildings and
transport. Missing is the life people are likely to lead, and the economy in which they are to
participate.
Somewhere there needs to be a vision of the town that London is to become – in terms of an
economy, a society, as well as a place.
Future economic and social changes
 London as a global city and a focus for sovereign wealth funds
 London as by far the largest city in Europe
 Impact of IT on the role of cities in general as a “middle man” that can be replaced by IT.
Potential for yet more jobs to be replaced by IT, from trading through to architectural design using
BIM.
 European/International economic and housing policy - Eg Polish cities in decline, British Cities
expanding
 Changes in demand and management for transport through Intelligent Transport Systems, smart
phones. – and cycling!
 Aging population
 Reduction in local authority resources – increase in demands on their resources with projected
increases in child and social care costs increasing child
A right and wrong way to design?
Can you produce a robust blue print on density and design?
No you can’t
o There is no right or wrong when it comes to urban design - you have to consider things
on their merits. Guidance can say what to consider and how, but not that a certain density
in a certain place will automatically be right or wrong.
Yes you can
o Lessons have been learned over the past 200 years and over the last 50 years in
particular: we know that there are some types of design that don’t work well.
Yes you must
o Its recognised that cities make us sick, with higher rates of suicide and ill-health
o Uncontrolled development lead to costs being imposed on others including overcrowding;
overloading of transport systems and local schools, and medical centres. (social
infrastructure) and environmental damage.
Ways forward
Recommendations for living at higher density are given in Superdensity published by Design for
Homes.
 London vernacular that does not depend on a cladding system
 Obvious front door
 Minimising the number of people using the same shared access to reduce anonymity and enable
self-policing
 Street based design
 Adequate parking provision – see Space to Park
New Density Policy the PTAL/general location type/density range policy is not being applied and
does not seem of much use. The superdensity report issues could form the basis of a criteria based
assessment regime for developments over a certain density range.
Research should be carried out to understand appropriate neighbourhood densities, their relationship
to local facilities and amenities to be used by residents, and the role of individual sites in supporting
these.
Consider a “Green finger” model of development instead of the green girdle – extended out over
existing transport routes, but constrained in width so that no one lives more than a mile from open
countryside.
Needs based housing – built to acceptable standards - We need a housing programme based on
needs, and built to standards.
Quality of development matters immensely, any matrix or density figure approach needs to be
complemented by considering quality of design and social impact (including impact on local schools
etc
Skills - Local authorities need to increase their urban design skills resources, or possibly use what
they have in a more strategic way, prioritising the use of that skill on the largest most important
applications, rather on small schmes.