Page 88 - Hillingdon SuDS DESIGN & EVALUATION

Detailed Design 9.5.4.7 Accounting for Urban Creep

Urban Creep considers the potential impact
on the drainage system from permitted
development such as paving over front
gardens to create driveways. Permitted
development rights generally applies to

residential development but can also apply to
commercial development and schools.

The following table is taken from LASOO
Guidance document and defines the
anticipated percentage increase to
impermeable area:

Paving over front gardens with impervious
surfaces is increasingly common. This example
could easily have been permeable block paved.

Residential development density
(dwellings per hectare)
83

≤ 25 30 35 45 ≥ 50 flats & apartments

Percentage area increase
applied as percentage of
proposed impermeable area 10% 8% 6% 4% 2% 0%
within curtilage of private
lands.

For housing developments designers should
calculate the number of properties per
hectare and apply the percentage increase to
non-adopted impermeable areas, for example
roofs, pathways and driveways.

Urban creep allowance for commercial
developments and schools should be agreed
with the LLFA at pre-application stage.

London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates