Is a 2m extension worth it?
On a Victorian terrace, often yes — but not for the floor area. A 2m rear extension combined with a side return infill squares off the ground floor, and it is that change in shape, plus the light from a glazed roof, that transforms the space rather than the 2m itself.
Two metres of depth on its own adds roughly 8 to 10m² to a terrace, which is a modest gain for the fixed costs involved — foundations, structural opening, party wall award and building control cost much the same whether you go out 2m or 4m. Judged purely on cost per square metre, a 2m extension is the least efficient size.
What changes the calculation is combining it with a side return infill. Filling the return and going out 2m together turns an L-shaped ground floor into a rectangle, which is a different room rather than a slightly bigger one. Most of the benefit people attribute to extending comes from that squaring-off and from the daylight a glazed roof brings into the middle of the plan.
There is also a planning argument for stopping at 2m or 3m: it stays inside standard permitted development on a terrace, avoiding both the prior approval process and a full application. If the garden is small, keeping it short also protects the outdoor space, which on a Fulham terrace is usually in short supply and worth more than the extra two metres inside.
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