Extending Your Period Home

When extending your period home to add one or more extra rooms, there are legal as well as practical and financial implications. Here we outline the law as it applies to house extensions in the United Kingdom.

We may buy a period house that meets our needs today, but then our requirements change. The house you buy when you are single may be too small if you start a family. Perhaps you become self-employed and work from home so you now need space for a home office. Or perhaps the house you originally bought was the largest you could afford and because the plot is large enough, now you can add an extension. Victorian and Edwardian houses, with their pitched roofs, often suitable for a loft conversion.

When considering a project such as an extension, a loft conversion, or even changing a garden for off-road parking, there are restrictions on what changes you may make.

The rules governing which domestic extensions can be classed as 'Permitted Developments' changed on 1st October 2008. The changes mean that far more householders who plan to add space to their property are removed from the planning process.

The most significant change is the removal of the rule which ties the volume of any new extension to the volume of the original house. Instead, extensions will be permitted so long as they fall within a clearly defined set of guidelines.


  • Extensions measuring up to 4 metres deep are permitted to detached houses. This is reduced to 3 metres on semi-detached and terraced properties.
  • The maximum height of a single-storey rear extension is 4 metres.
  • For extensions of 2 or more storeys, the maximum depth will always be 3 metres, including the ground floor.
  • Where an extension falls within 2 metres of a boundary, the maximum eaves height is 3 metres.
  • The eaves and ridge height of any extension should be no higher than those of the existing house.
  • Extensions to the side of a property can be single storey only and should be no more than half the width of the original house.

Loft Conversions

It was decided that volume caps should be retained for loft conversions to avoid the construction of over-bearing dormers. However, the allowances should make it possible for most householders to add a full width rear dormer or to build up a gable end in place of a hipped roof structure.

The main guidelines governing permitted Development to roof spaces are:

  • The additional volume added is capped at 50 cubic metres for detached and semi-detached houses. For terraced properties, the volume allowance is reduced to 40 cubic metres.
  • Dormers are not permitted to any elevation that fronts on to a highway.
  • Dormers should be set back from the eaves by at least 20cm and should not extend above the ridge line of the existing roof structure.

References to the 'original house' mean the house when it was first built, or on 1st July 1948 if it was built prior to that date.

Conservation Areas

Further restrictions still apply to properties within designated Conservation areas:

  • Dormers are not permitted
  • Rear extensions can be single storey only
  • No side extensions are permitted.

If a proposed extension does not comply with the guidelines above it will not necessarily be refused but it will require the submission of a planning application. All extensions to listed buildings will require listed building consent.

A Building Regulation completion certificate continue to be required on all extensions and if the planned works fall within the scope of the Party Wall etc. Act 1996 there is a requirement to serve notice on all affected adjoining owners.


The Government is also attempting to use Permitted Development Rights to tackle the growing problem of rainwater running off paved-over front gardens. Research has shown that this phenomenon has contributed significantly to the increased instances of flooding in recent years.

Paving to driveways covering an area of five square metres or more are only be permitted if permeable materials such as gravel or open jointed paving stones are used.

Article by Justin Burns BSc MRICS of Peter Barry Chartered Surveyors and Party Wall Consultants