Permitted development is not a size, it is a set of tests, and the answer for your house depends on things a general article cannot know: what the house was when first built, whether anything has been added since, whether the rights have been removed locally, and whether the property is a house at all. What follows is what the tests measure, so you can read the guidance against your own plot.
What the permitted development extension rules measure
Depth beyond the original rear wall, overall height and eaves height, whether the addition passes beyond a side elevation, proximity to the boundary, how much of the garden is covered, and the materials. The limits differ between a detached house and everything else, and they tighten in conservation areas, national parks and other designated land. A separate procedure covers deeper single storey rear extensions, where the council notifies the neighbours and decides before work starts.
The original house, and what has already been spent
The allowance belongs to the house as originally built, or as it stood at the historic date the rules use, not as you bought it. Anything a previous owner added has already consumed part of it. This is the most common reason a project believed to be permitted development turns out not to be, and it is discoverable in advance from the planning history on the council's own website.
Double storey extension planning permission and kitchen extensions
Two storey additions have their own tighter limits on depth and on distance from the boundary, and are the ones most likely to need an application. A kitchen extension is judged as an extension like any other, so the answer is about size and position rather than about the room it contains. What changes with a kitchen is the building regulations side: drainage, ventilation and extraction all engage whatever the planning position.
Getting it in writing, and how long it takes
Where the work is permitted development, a lawful development certificate is how you hold that in writing, and a buyer's solicitor will ask for it. Where an application is needed, householder applications have a statutory determination period, and delays usually come from validation, from consultation or from a case officer asking for changes rather than from the clock itself. Submitting a complete, valid application is the single best way to shorten it.
Questions people ask about extension size without planning permission
What size extension can I build without planning permission UK-wide?
No. The limits depend on house type, the original building, the boundary, the location and what has been added before.
Do permitted development rights apply to flats?
No. Householder permitted development rights apply to houses, so a flat or maisonette needs an application.
How long does planning permission take for an extension?
There is a statutory determination period for householder applications. Validation problems and consultation are what usually stretch it beyond that.
What is a lawful development certificate for?
It is formal confirmation that what you built, or propose to build, is lawful without permission. It is the document that protects you on sale.