Which direction you build is a bigger decision than how far. Going sideways into a side return widens a narrow room and usually keeps the garden. Going backwards deepens the plan and usually darkens the middle. Wrapping around does both and costs more than the sum of the two, because the corner is where the structure, the roof and the drainage all meet.
The side return: width where it is missing
On a terraced or semi detached house with a rear projection, the strip of ground beside it is often too narrow to use for anything. Roofing it over turns a corridor kitchen into a room. Because the new width sits against a boundary, this is the option most likely to engage the party wall procedure and most likely to need careful attention to the neighbour's light and drainage. The gain per pound spent is usually the best of the three.
The rear kitchen extension: depth, and what it costs the old rooms
Building straight back is simpler to build, easier to roof and normally cheaper per square metre. Its cost is paid by the original rear room, which loses its window to an internal opening. Rooflights over the old boundary line, or removing that wall entirely with a beam, are the two ways round it, and both should be decided at sketch stage rather than after the foundations are set out.
A wrap around kitchen extension and the corner
An L shaped wrap gives the width and the depth and produces the most flexible plan, but the internal corner is a genuinely difficult junction: two roof planes meet, the drainage has to get past, and the structure usually needs a post or a goalpost frame where the two openings intersect. Budget for the corner rather than discovering it.
A Victorian terrace kitchen extension and older stock constraints
In Victorian stock expect solid walls with no cavity, shallow foundations, a drainage run in an awkward place and often a manhole exactly where you want the new wall. Building over or near a public sewer is a separate consent from the water company, and finding it late is one of the more expensive surprises in domestic building. In a conservation area the rear is usually more forgiving than the front, but the roof form and the glazing detail will still be looked at.
Questions people ask about side kitchen extension
Which gives more usable space, side or rear?
On a narrow terrace the side return usually wins, because the width it adds is width the room never had. On a wide plot the rear normally wins.
Does a side return extension always need a party wall notice?
Often, because the work is on or near a boundary or involves excavation close to a neighbour's foundations. Read the explanatory booklet and serve notice in good time.
Why is a wrap around more than twice the price?
Because of the corner. Two roofs, two sets of structure and a drainage route all have to resolve in one place, and that junction takes design and site time.
Are glass kitchen extensions comfortable?
Only if it is shaded and ventilated. Large glass areas without solar control give a room that is unusable on a hot afternoon and expensive to heat in winter.