Most cost guides give you one number. “An extension costs around fifty thousand.” That number is almost useless because it hides more than it reveals. Build cost or total cost. Which type of extension. What specification. What about the fees, the fitout, the finishing?
Here is the real breakdown. Every line item. Every cost most guides leave out. The numbers that croydon architects and honest builders will confirm but that rarely appear in a single transparent list. This is what an extension in Croydon actually costs in 2026.
The Build Cost
This is the number people quote. The amount the builder charges to construct the extension.
Single-story rear extension. Three meters deep. Standard specification. Thirty-five to forty-five thousand.
Single storey rear extension. Four meters deep with the larger permitted development scheme. Forty five to fifty five thousand.
Double storey extension. Two floors of new space. Sixty five to eighty five thousand. The most cost effective way to add significant space because you dig foundations once and build two floors on them.
Side return infill on a Victorian terrace. Twenty five to thirty five thousand. Less than a full rear extension because it uses the existing boundary wall.
These are build costs only. The builder’s invoice. Roughly sixty to seventy percent of what you actually spend.
The Professional Fees
Architect. Eight to twelve percent of the build cost for a full service. On a fifty thousand build that is four to six thousand. Design, planning application, building regulations drawings, and site visits.
Structural engineer. Fifteen hundred to two and a half thousand. Foundation design, beam calculations, structural details. Sometimes included in the architect fee. Usually separate.
Building control. Three to five hundred for the application. Plus inspection fees. Either through the council or a private approved inspector.
Planning application fee. Around three hundred for a householder application. Nothing if the project qualifies for permitted development through a lawful development certificate which costs about a hundred and fifty.
Total professional fees. Six to nine thousand on a typical project.
The Party Wall Costs
If your extension affects a shared wall or excavates near a neighbours boundary, the Party Wall Act applies.
Your surveyor. Fifteen hundred to two thousand. If the neighbour consents to your notice, one surveyor handles it.
If the neighbour dissents and appoints their own surveyor, you pay for both. Three to four thousand combined.
Most projects with attached neighbours need party wall agreements. Budget two to four thousand depending on how many neighbours are affected and whether they consent.
The Kitchen Fitout
The extension creates the space. The kitchen fills it. And the kitchen is a separate cost that no builder quote includes.
Budget kitchen. Units, worktops, and appliances. Eight to twelve thousand.
Mid range kitchen. Better units, quartz worktops, integrated appliances. Twelve to eighteen thousand.
Premium kitchen. High end units, stone worktops, premium integrated appliances, boiling water tap. Eighteen to thirty thousand.
The kitchen fitout is the single biggest cost that homeowners forget to budget for. They remember the build cost and assume the kitchen is included. It never is.
The Finishing Costs
Flooring. Two to five thousand depending on material. Engineered oak costs more than vinyl. Tiles cost more than laminate.
Decoration. A thousand to two thousand. The builder usually allows basic white emulsion. Anything beyond that is extra.
Landscaping. One to three thousand. The builder destroys the garden during construction. Reinstating the lawn, repairing the patio, and replacing damaged fencing is your cost.
Lighting and electrics beyond basic. Five hundred to fifteen hundred. Additional circuits, better fittings, under cabinet lighting, external sockets.
Total finishing costs. Four to eleven thousand.
The Contingency
Every project should have a contingency. Five to ten percent of the total project cost. For genuine unknowns. A drainage complication. A material substitution. Something nobody could predict.
On a project totalling seventy thousand a contingency of five to seven thousand is sensible. Most of it stays unused if the project is well planned. But having it prevents panic when a small surprise emerges.
The Total Real Cost
A three metre single storey rear extension in Croydon in 2026.
Build cost. Forty thousand. Professional fees. Seven thousand. Party wall. Two thousand. Kitchen fitout. Fifteen thousand. Finishing. Six thousand. Contingency. Five thousand.
Total real cost. Seventy five thousand.
Not the forty thousand build figure that most people quote. Seventy five thousand all in. Build cost is roughly fifty three percent of the total. The other forty seven percent is everything people forget to budget for.
Why the Real Number Matters
Homeowners who budget for the build cost get blindsided by the rest. They borrow or save forty thousand. Then discover they need seventy five. They cut corners on the kitchen to afford the build. They skip the contingency and panic at the first surprise. They run out of money before the project finishes.
Homeowners who budget for the real total plan properly. They know the full cost before they start. They fund the kitchen they want rather than the kitchen they can afford after the build consumed everything. They have a contingency that absorbs surprises without stress.
The difference between these two homeowners is not income. Its information. Knowing the real number at the start.
What to Ask Your Architect
At the first meeting ask for the total project cost. Not the build estimate. The total. Including professional fees, party wall, kitchen fitout, finishing, and contingency.
A good architect gives you this number honestly even though it is higher than you want to hear. A weak architect gives you the build cost and lets you discover the rest through a series of unexpected invoices.
The honest number is higher. But it is the truth. And the truth at the start beats a nasty surprise halfway through every single time.
Six to eight months from first conversation to completion. Seventy five thousand for a three metre extension done properly. Budget for the real number not the build number.
