The “original house” is the house as it stood on 1 July 1948, or as first built if it was built later. Earlier owners' extensions and roof enlargements can still affect what you may add; buying the property does not reset the allowances. [1]
Original and existing mean different things
The existing house is the building immediately before the proposed development. It can include additions which are not part of the original house. Permitted development tests use these terms deliberately, so the current back wall is not necessarily the correct starting line for a rear-extension measurement. [1]
For example, suppose a house has a later rear addition and you propose to enlarge it again. Do not measure the new projection only from the back of that addition. The original rear-wall position and the treatment of the combined enlargement must be established. This is an illustration of the government's measurement guidance, not a new allowance. [1]
There is more than one allowance to check
The depth and form of an extension
Rear-extension limits relate to the original rear wall. Where a new extension joins an existing enlargement, the enlarged part must be assessed as a whole under the relevant limitations. A previous addition with planning permission is not automatically left out of that assessment. [1][2]
Coverage within the curtilage
The householder rules include a limit on the proportion of the curtilage covered by buildings other than the original house. Existing extensions and outbuildings matter to that calculation. It is not simply a question of whether the proposed extension alone occupies less than half the present garden. [2][1]
Roof enlargement
Roof enlargements have their own cumulative volume test. The familiar limits are 40 cubic metres for a terraced house and 50 cubic metres for other houses, with previous roof-space enlargements counted, including those undertaken by earlier owners. Those figures are not a substitute for the remaining roof-development conditions. [1]
Establish the baseline before fixing the design
Use the planning history and available historic drawings to build a chronology: original building, later extensions, roof changes and the current proposal. Label what each document establishes and distinguish an approval drawing from evidence of what was actually built. The relevant definitions concern the building at the specified time, not merely a drawing someone once obtained. [1][3]
If the original arrangement remains uncertain, identify the missing evidence explicitly. A proposed Lawful Development Certificate requires enough information for the council to establish that the described works would be lawful; an unsupported assumption about the baseline can prevent a positive determination. [3]
Does a previous planning permission use up everything?
Not necessarily. Different rights have different tests, and an earlier permission is not a blanket cancellation of all future permitted development. However, its conditions may remove particular rights, and the physical additions can affect later dimensional or cumulative calculations. Read both the decision notice and the building history. [4][5]
The useful outcome is a dimensioned “original / existing / proposed” drawing set. It allows the assessment to explain exactly what is being counted instead of describing the property as having an unspecified amount of permitted development left.