How much does a side extension actually cost?
Side extension costs vary hugely depending on scope — a simple bolt-on extension is a very different job from an infill extension that involves removing a full side wall and reconfiguring the rear of the house around it. To show real numbers rather than a vague range, here’s a genuine Fluent project costing, broken down by area.
The project
A large Victorian semi in Ealing, west London — five bedrooms, with the typical period layout of lots of small, separate rooms: a dark corridor running the length of the party wall, connecting a cramped kitchen to a large but disconnected living room at the rear.
The brief was an infill side extension, roughly 2.4 metres wide by 8 metres long, which meant removing the entire side wall of the house and replanning the whole rear layout around it — not a bolt-on, but a full reconfiguration knitting the new side infill into a rebuilt kitchen/living/dining space, a boot room, and a downstairs WC.
Why costs vary so much between builders
When we cost a project, we don’t produce a single number. Our costing software pulls labour and material rates from different builders working in the same area, which naturally cluster into low, mid, and high bands — different build price scenarios depending on which builder ends up doing the work. Alongside that, we also produce our own Fluent figure, based on the rates and specification we design to.
For this project, isolating just the side extension and the ground floor rooms it directly affects (side extension, kitchen/living/dining, boot room, downstairs WC, and general ground floor works), the totals came to:
Cost scenario | Total (ex VAT)
Low – £123,929
Mid – £138,575
High – £152,095
Fluent – £118,162
That’s a gap of nearly £34,000 between the low and high bands, on the same brief, same drawings, same specification. That’s exactly why “how much does a side extension cost” doesn’t have one answer — it depends heavily on who’s pricing it, not just on square metres.
Where the money goes
The side extension shell itself – (structure, roof, walls) is the single biggest line item, but not the majority of the cost
The kitchen/living/dining space — fully reconfigured, not just extended into — carries a large share of the budget: new floor and wall finishes, units and appliances, heating, and decoration across a much bigger footprint than before
Smaller connected spaces (boot room, WC) add up more than people expect, because even a small room needs the same categories of work — plumbing, electrics, finishes, doors — as a large one
This is the pattern we see repeatedly on infill side extensions: people budget for the extension and underestimate how much the *knock-on* work in the existing rooms costs. Removing a full side wall and replanning the rear of the house means paying to rebuild the rooms either side of it too, not just the new floor space.
What this means for your budget
For a side extension of similar scale — full side wall removal, rear reconfiguration, rather than a simple bolt-on — a working range of roughly £120,000–£150,000 ex VAT is a realistic starting point, before professional fees, planning costs, and contingency on top.
The exact figure will vary by house and builder. The shape of where the cost sits is the useful part: the extension shell is significant, but the reconfigured existing rooms around it often cost just as much, if not more.
