Kitchen extensions are sold by depth and lived in by layout. The metre you add at the back is only useful if it lands where the work happens, and past a certain depth the extra floor buys a darker middle rather than a better kitchen. The right size is the one that gives a working triangle, a table that people can get around, and a route to the garden that does not run through the cooking.
What each metre buys, and why a 3m kitchen extension is common
The first couple of metres usually buy a proper run of units and a hob wall. Around three metres is where a table stops being an obstruction. Beyond that you are buying seating or a sofa rather than kitchen, and you are also buying a deeper plan whose middle gets darker. This is why so many rear extensions land at a similar depth: it is where the useful gain flattens out, not a rule anyone wrote.
Small kitchen extensions, a large kitchen extension, and the trade in between
A small kitchen extension that simply squares off a side return can transform a narrow Victorian plan without a large budget, because it widens the part of the room that was unusable. A large one buys a genuinely different way of living, at the price of heating a bigger volume, lighting a deeper plan and losing more garden. The middle option, going wide rather than deep, is underused and is usually the better value where the plot allows it.
Open plan kitchen extensions have real consequences
Cooking smells, noise and heat move freely in an open plan. Extraction has to be sized properly and ducted to outside rather than recirculated if the kitchen is genuinely part of the living space, and the ventilation guidance sets the floor. Sound carries further than people expect, which is the most common regret. A single closable door somewhere in the plan solves most of it and costs very little at design stage.
A vaulted ceiling kitchen extension, glazing and overheating
A vaulted ceiling over the new space adds volume and light and makes a modest extension feel considerably larger, at the cost of more structure and a warmer room in summer. Large areas of south facing glass and rooflights do the same. The overheating guidance is now part of the approved documents for new dwellings and the principle is worth applying anyway: shade, cross ventilation and the ability to open something at both ends.
Questions people ask about kitchen extensions
How deep can a kitchen extension go without planning permission?
Depth limits under permitted development depend on the house type and whether the larger home extension procedure is used, so check the technical guidance and your own property's history before assuming.
Is open plan a mistake?
Not in itself. The mistake is having no way to close anything off. Broken plan, a pocket door or a separate snug keeps the openness and fixes the noise.
Does a vaulted ceiling need more structure?
Usually yes, because the roof can no longer be tied across at ceiling level, so the structure carries the spread differently. It is a design decision to take early.
What ruins a new kitchen most often?
Under specified extraction and a dark middle. Both are cheap to design out and expensive to fix afterwards.