Hillingdon - DMHD 1: Alterations and Extensions to Residential Dwellings

Policy information sourced from the Hillingdon Local Plan: Part 2

DMHD 1: Alterations and Extensions to Residential Dwellings

Planning applications relating to alterations and extensions of dwellings will be required to ensure that:

there is no adverse cumulative impact of the proposal on the character, appearance or quality of the existing street or wider area;
a satisfactory relationship with adjacent dwellings is achieved;
new extensions appear subordinate to the main dwelling in their floor area, width, depth and height;
new extensions respect the design of the original house and be of matching materials;
there is no unacceptable loss of outlook to neighbouring occupiers;
adequate garden space is retained;
adequate off-street parking is retained, as set out in Table 1: Parking Standards in Appendix C;
trees, hedges and other landscaping features are retained; and
all extensions in Conservation Areas and Areas of Special Local Character, and to Listed and Locally Listed Buildings, are designed in keeping with the original house, in terms of layout, scale, proportions, roof form, window pattern, detailed design and materials.

Rear Extensions

single storey rear extensions on terraced or semi-detached houses with a plot width of 5 metres or less should not exceed 3.3 metres in depth or 3.6 metres where the plot width is 5 metres or more;
single storey rear extensions to detached houses with a plot width of 5 metres or more should not exceed 4.0 metres in depth;
flat roofed single storey extensions should not exceed 3.0 metres in height and any pitched or sloping roofs should not exceed 3.4 metres in height, measured from ground level;
in Conservation Areas and Areas of Special Local Character, flat roofed single storey extensions will be expected to be finished with a parapet;
balconies or access to flat roofs which result in loss of privacy to nearby dwellings or gardens will not be permitted;
two storey extensions should not extend into an area provided by a 45-degree line of sight drawn from the centre of the nearest ground or first floor habitable room window of an adjacent property and should not contain windows or other openings that overlook other houses at a distance of less than 21 metres;
flat roofed two storey extensions will not be acceptable unless the design is in keeping with the particular character of the existing house;
pitched roofs on extensions should be of a similar pitch and materials to that of the original roof and subordinate to it in design. Large crown roofs on detached houses will not be supported; and
full width two storey rear extensions are not considered acceptable in designated areas or as extensions to Listed Buildings or Locally Listed Buildings.

Side Extensions

side extensions should not exceed half the width of the original property;
extensions to corner plots should ensure that the openness of the area is maintained and the return building line is not exceeded;
garages should reflect the size guidelines set out in Appendix C Parking standards;
two storey side extensions should be set in a minimum of 1 metre from the side boundary or in the case of properties in the Copse Wood and Gatehill Estates, at least 1.5 metres, but more if on a wider than average plot, in order to maintain adequate visual separation and views between houses;
two storey side extensions to detached and semi-detached properties should be set back a minimum of 1 metre behind the main front elevation;
where hip to gable roof extensions exist, a two storey side extension will not be supported; and
in Conservation Areas, single storey side extensions may be required to be set back.

Front Extensions

alterations and extensions to the front of a house must be minor and not alter the overall appearance of the house or dominate the character of the street. Front extensions extending across the entire frontage will be refused;
porches should be subordinate in scale and individually designed to respect the character and features of the original building; pastiche features will not be supported; and
notwithstanding the above, at least 25% of the front garden must be retained.

Roof Extensions

roof extensions should be located on the rear elevation only, be subservient to the scale of the existing roof and should not exceed more than two thirds the average width of the original roof. They should be located below the ridge tiles of the existing roof and retain a substantial element of the original roof slope above the eaves line;
the Council will not support poorly designed or over-large roof extensions including proposals to convert an existing hipped roof to a gable;
raising of a main roof above the existing ridgeline of a house will generally not be supported;
all roof extensions should employ appropriate external materials and architectural details to match the existing dwelling; and
in Conservation Areas, Areas of Special Local Character and on Listed and Locally Listed Buildings, roof extensions should take the form of traditional ‘dormer’ windows, on the rear elevation, to harmonise with the existing building. The highest point of the dormer should be kept well within the back roof slope, away from the ridge, eaves or valleys, whilst each window should match the proportions, size and glazing pattern of the first floor windows.

Front Gardens and Parking

new or replacement driveways should use permeable (porous) surfacing. Surfaces of more than five square metres will need planning permission for laying traditional, impermeable driveways; and
the design, materials and height of any front boundary must be in keeping with the character of the area to ensure harmonisation with the existing street scene.

For more information please see the Local Plan: Part 2