Extension Costs
What extensions actually cost in Colchester, what drives the number, and what is never included in a per square metre figure.
Real ranges, with what they include, what they exclude, and what actually moves the number. Anyone giving you a single figure without seeing your house is guessing, and usually low.
Indicative ranges
These are broad guides for the Colchester area, current at the review date at the foot of this page. They are not a quotation. What your job costs depends on the ground, the access, the structure and the specification, and those are assessed at survey.
| Type | Typical range per m² | Notes |
|---|---|---|
| Single storey extension | £2,200 to £3,000 | Shell and standard finish. Deep foundations push higher |
| Two storey extension | £1,900 to £2,600 | Lower per m²: two floors from one set of foundations and one roof |
| Side return extension | £2,600 to £3,400 | Access through the house and boundary work add cost |
| Loft conversion, dormer | £1,500 to £2,200 | No foundations. Part B staircase work is the variable |
| Garage conversion | £1,100 to £1,700 | Walls and roof already exist. Floor and damp are the work |
| Orangery | £2,800 to £3,800 | Masonry piers and a lantern roof, built to full Building Regs |
To put that in a whole number: a 20m² single storey rear extension, which is a typical kitchen and dining space, commonly lands somewhere between £44,000 and £60,000 for the shell and standard finish, before the exclusions below. A 30m² wrap around on a terrace with no side access can be considerably more.
What is included
- Groundworks and foundations to the specified depth, and disposal of spoil
- Drainage connections and alterations
- Structure, including the steel beam, padstones and temporary support
- External walls, insulation and roof
- Windows and external doors to the specification quoted
- Plastering, first and second fix carpentry
- Electrics and plumbing to a standard specification
- Making good where new work meets old
What is excluded, and should be budgeted separately
- VAT
- Kitchen units and appliances, and bathroom sanitaryware
- Flooring and decoration
- Professional fees: architectural drawings and structural calculations
- Statutory fees: planning or prior approval, Building Control, Lawful Development Certificate
- Party wall surveyor fees where a neighbour dissents
- Build over agreement with the water authority, where one is needed
- Service diversions, and anything genuinely unforeseeable found once the ground is open
Allow a realistic figure for fees. On a typical extension, drawings, structural calculations and statutory fees together are commonly a few thousand pounds, and they are payable whoever builds it.
What moves the number most
Foundation depth. The largest single variable around Colchester, because most of the ground is shrinkable clay and mature trees are common. A one metre dig and a two and a half metre dig are not the same job, and near a large oak a piled foundation with a suspended slab becomes the sensible answer. This is the difference that explains most suspiciously cheap quotes.
Access. A house with a side gate is materially cheaper to build on than a terrace where every barrow goes through the hallway on boards. Nothing about the extension changes, but the labour does.
Structural span. A wide opening needs a bigger steel, heavier padstones and sometimes load taken down to new foundations. The depth of a beam is set by span and load, and it is not negotiable.
Glazing. The range between standard windows and a full width run of high specification bifolds or sliders with solar control glass is tens of thousands of pounds on its own, and it is the item most often left vague in a cheap quote.
Roof type. A warm deck flat roof, a pitched roof, or a lantern each cost differently and each suit different situations.
Specification. Underfloor heating, rooflights, level thresholds and finish level all move the total. None of them are hidden, but they need to be decided before pricing rather than added afterwards.
How we price
We survey first, always. We look at the ground, the trees, the access, the existing structure, the drain runs and the planning position, and then we price the job we have actually seen.
Where we make an assumption, the quotation says so, and it says what happens if the assumption turns out to be wrong. Foundation depth is the usual one.
The full account of that process, from the site visit to what lands in your inbox afterwards, is on how our quotes work.
We work to stage payments against completed work, set out in the quotation. We do not ask for a large payment before anything has happened. Where something unexpected is found, we stop, show you, explain the options and price the change before carrying it out, and you approve it in writing.
A note on comparing quotes
The cheapest number is frequently the least complete one. Before comparing, check that each quote states the foundation depth allowed, includes or excludes structural calculations, names the Building Control route, and lists its exclusions. A quote with no exclusions section is an opening position rather than a price.
Ask what the mechanism is if the ground turns out worse than assumed. Every honest builder has one. Not having one means the risk is being carried silently, and it will surface later.
Last reviewed August 2026.