Most expensive extension mistakes are design and contract decisions made before work starts: building out so far that the original rooms go dark, no contingency in the budget, a quote with no written specification so variations are unpriced, ignoring a drain run or a neighbour's rights, and leaving the kitchen layout and services to be decided on site.
- An extension that darkens the room it joins makes the house worse. Plan the light, not just the footprint.
- Hold 10 to 15 per cent contingency. Groundworks is where the surprises live.
- A quote without a specification cannot be compared and cannot be held to.
- Locate the drains before the design is fixed, not after the trench is open.
The mistakes that cost the most
These are listed roughly in order of what they cost, which is not the order people worry about them in. The expensive ones are decided on paper.
- Darkening the existing rooms. A deep rear extension puts the original back room in permanent gloom. This is the single most common regret and it is entirely a design problem.
- No contingency. Groundworks reveals things. Without 10 to 15 per cent held back, the first surprise forces a compromise somewhere visible.
- A quote with no specification. If the quote does not say what is included, every question later is a variation at the builder's price.
- Ignoring drainage. A drain run under the footprint means a diversion, a build-over agreement or a redesign. Found late, it is expensive.
- Skimping on the structural opening. The steel that connects old to new decides whether you get a clean open space or a bulkhead across the ceiling.
- Deciding the kitchen after the build. Sockets, waste, extraction and lighting all need to be set before first fix.
- No party wall notice where one was needed, which can stop the job.
- Choosing on price alone, which selects for whatever was left out of the cheapest quote.
Light: the mistake people regret longest
Worth its own section, because it is the one that cannot be fixed afterwards.
A single-storey rear extension takes light from the room it attaches to. The original back room had a window in an external wall; now that wall is internal, and whatever light reaches it comes through the new space. Build out four metres with a flat roof and standard doors at the far end, and the original room becomes a corridor.
What prevents it:
- Rooflights over the junction, not just at the far end. Light dropping where the old wall was is what keeps the original room usable.
- A structural opening as wide as the structure allows, so light travels through rather than past a narrow doorway.
- Thinking about depth against width. A wider, shallower extension often gains more usable space than a deep narrow one, and keeps the light.
- Glazing on the side elevation where the boundary allows, which is often more useful than more glass at the end.
- Pale finishes and a continuous floor between old and new, which makes the light go further.
- Standing in the room at different times of day before the design is fixed, and asking where the light will come from.
The test is simple: for every part of the extended space, name where its daylight comes from. If any part has no answer, the design is not finished.
The quote: what has to be in it
A builder's quote that is a single figure with a one-line description is not a quote you can compare, hold to, or sensibly accept.
What should be specified:
| Item | What to look for | Why it matters |
|---|---|---|
| Foundations | Assumed depth, and what happens if deeper | The commonest source of extra cost |
| Spoil and muck away | Included or excluded, and how many loads | Routinely excluded; substantial |
| Structural steel | Size, who designs it, who installs | Decides whether you get a bulkhead |
| Drainage | Diversion, connections, build-over | Unknown until the drains are located |
| Windows and doors | Make, spec, glazing, colour | Enormous price range |
| Roof build-up | Insulation, membrane, finish, falls | Where flat roof quality is decided |
| Electrical | Number of points, and who specifies | Vague here means variations later |
| Plastering and decoration | Included or not; where it stops | Often ends at the new walls only |
| Making good to existing | Explicitly stated | The junction is always more work than expected |
| Floor finish | Screed, insulation, and the finish itself | Finishes often excluded |
| Building control | Who applies, who pays the fee | Small but it causes arguments |
| Programme and payment stages | Dates and what triggers each payment | Protects both sides |
Two quotes that differ by a substantial margin almost always differ in what is in this table rather than in the builder's margin. Getting both to specify the same list is what makes them comparable — and the exercise frequently closes most of the gap.
Drainage and the neighbour: check both early
Find the drains before the design is fixed
Lift the inspection chamber covers and work out where the runs go. A public sewer or a shared drain within three metres of the extension generally needs a build-over or build-near agreement with the water authority, which takes time. A private drain under the footprint needs diverting, rerouting or bridging with a lintel. All of those are manageable when they are designed in, and all of them are disruptive and expensive when the trench is already open and the groundworker has found a pipe.
Serve the party wall notice
Where the work involves cutting into a shared wall, building up to or astride the boundary, or excavating within three metres of a neighbouring structure and below its foundation level, notice is required. The neighbour can consent or require a surveyor and an award. Serving notice early costs nothing but time; not serving it can mean an injunction stopping the work. On a semi or a terrace this applies far more often than people expect.
Talk to the neighbour anyway
Separate from any legal requirement. They will live with the scaffold, the skip, the deliveries and the noise, and a neighbour who was told in advance is a neighbour who complains to you rather than to the council. Where the extension affects their light or outlook, hearing it from you first is considerably better than seeing a notice.
Check the permitted development limits properly
Depth, height, eaves height, boundary proximity, materials and how much of the garden is covered all have limits, and a previous extension counts against your allowance. Whether you need permission is covered separately. Where you are relying on permitted development, a lawful development certificate is worth having, because it is the document that answers a buyer's solicitor years later.
Decide the inside before first fix
The build proceeds in a fixed order, and a decision made after its moment has passed costs several times what it would have cost in time.
Before the slab: underfloor heating, insulation depth, floor build-up and therefore finished floor level, and any waste runs in the floor.
Before first fix: the kitchen layout, because it determines socket positions, waste and water for the sink and appliances, extraction routing, and lighting positions. Also the position of any island, which needs services in the floor.
Before plastering: every socket, switch, data point, aerial, speaker and light position. Plus noggins for anything wall-mounted — a television, a boiler, heavy shelving.
Before the roof closes: any rooflights, extraction penetrations and the ventilation strategy.
A useful discipline is to draw the furniture on the plan. Where does the table go, which way do the doors open, where does the fridge sit, where will anyone plug a lamp in. The number of extensions with no socket where the lamp needs to be is remarkable, and it is a two-minute decision taken at the wrong time.
What it costs
These are the extension figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Build cost | £1,500–£2,500 per m² | Spec dependent |
| Single-storey extension | £20,000–£60,000 | Size & finish dependent |
| Double-storey extension | £100,000+ | Larger projects |
| Average project | ~£30,700 | Across typical jobs |
Hold 10 to 15 per cent of the figure back as contingency rather than committing all of it. Foundations and drainage are the two items that most often come in above assumption, and both are discovered in the first fortnight.
We quote against a written specification and locate the drains before the design is fixed, so the surprises happen on paper rather than in the trench.
Frequently asked questions
What is the most common extension mistake?
Darkening the room the extension joins. The original back room had a window in an external wall; once that wall is internal, its light comes through the new space. Build out deep with glazing only at the far end and the original room becomes a corridor. Rooflights over the junction and a wide structural opening prevent it.
How much contingency should I allow on an extension?
Ten to fifteen per cent, held back rather than committed. Foundations and drainage are the two items that most often exceed the assumption, and both are discovered within the first fortnight. Without contingency the first surprise forces a compromise on something visible and permanent.
What should be in an extension quote?
Assumed foundation depth and what happens if deeper, spoil removal, structural steel and who designs it, drainage and connections, window specification, roof build-up, number of electrical points, where plastering and decoration stop, making good to the existing house, floor finishes, building control fees, and the payment stages.
Do I need a party wall agreement for an extension?
Where the work cuts into a shared wall, builds up to or astride the boundary, or excavates within three metres of a neighbouring structure below its foundation level, notice is required. On a semi or terrace that applies more often than people expect, and not serving it can mean an injunction stopping work.
What happens if there is a drain under my extension?
It needs diverting, rerouting or bridging with a lintel, and a public or shared drain within three metres generally needs a build-over agreement with the water authority. All of that is manageable when designed in and expensive once the trench is open, so lift the chamber covers before the design is fixed.
When do I need to decide the kitchen layout?
Before first fix. The layout determines socket positions, waste and water for the sink and appliances, extraction routing and lighting positions, and an island needs services in the floor before the slab goes down. Deciding afterwards means chasing walls or accepting sockets in the wrong places.
Get the paper right first
We specify the foundations, drainage, steel and finishes in writing before starting, because that is where the expensive mistakes are actually made.