House extension size - are there any limits?

A house extension built under permitted development rights will have certain dimensional constraints that will limit the extension size.  House extensions that require formal planning consent are only limited in size by the opinion of the case planning officer and their interpretation of the relevant planning policy.

Most house extensions that require formal Planning permission do need to respect the scale of the original dwelling and must not be out of scale or disproportionate to the main dwelling.  There are exceptions to this when an extension is deliberately designed to transform the shape and character of a dwelling into something more appropriate to the surrounding area.

An example of this could be a poorly designed 1960's common brick box built within a conservation area.  A large full width front extension using clay brick and flint under a hand made plain clay tiled roof could define a new style of dwelling more sympathetic to its surrounding historic buildings.

However, in most cases a house extension  size will be limited by factors of scale and neighbourliness.  Planners will also be considering any precedents that approving a very large house extension scheme may have on future development's within the locality.

Planners are always mindful of being under pressure from a neighbour to do the same thing where the accumulative effect down the street could be very undesirable in adversely affecting a sites local character.

So, yes, there are limits to a house extension size but can only be based upon a site by site basis for each house extension proposal.  Your home extension designer or architect will be able to advise you on what they consider the maximum size of extension that should be approved under Planning permission or permitted development.