How much is a 4m by 5m extension in Teesside?

Roughly £38,000 to £52,000 for 20m² at local rates, including structure, glazing, services, plaster and floor finish, before kitchen units and appliances. Deep foundations near trees or a wide bifold run push it higher.

Do I need planning permission for a rear extension?

Often not. Up to three metres on a semi or terrace and four on a detached house usually falls within permitted development, with a prior approval route for larger single-storey builds. Two-storey work and conservation areas normally require a householder application.

How deep will the foundations need to be?

Around 900mm to 1.2m on typical local clay with no trees nearby, and 1.5m to 2.5m or a piled solution where mature trees stand close to the build. Building control sets the depth on site once the trench is open.

What is a build-over agreement?

Consent from Northumbrian Water to build over or within three metres of a public sewer. It is common in older Teesside streets, routine to obtain, and takes several weeks, so it should be applied for early.

Will I be without a kitchen for months?

Usually only for the last few weeks. The existing rear wall stays in place until the new structure is watertight and largely finished, so the break-through and kitchen fit happen at the end rather than the beginning.

Is a two-storey extension better value than a single storey?

Per square metre, yes, because the foundations and roof are already paid for. In absolute terms it costs more and is more likely to need planning permission, so the right answer depends on whether you need bedrooms as well as living space.

House extensions in Teesside

Rear, side, wrap-around and two-storey extensions across Teesside, priced at £1,900 to £2,600 per square metre for single storey and £1,700 to £2,300 for two. Quoted from measured drawings and a written schedule of works, with planning and building control run as part of the project.

Get Quote From £1,900/m²

Two-storey brick side extension under construction with scaffolding and new roof trusses

Which extension suits which house

The shape of the extension is decided by the shape of the plot far more than by taste. A rear extension is the default on Teesside semis and terraces, because back gardens here are usually deeper than they are wide, and it delivers the kitchen-diner most families are actually after. A side extension suits the many local semis with a driveway down one flank, converting three metres of tarmac into a utility, downstairs WC and a wider kitchen. A wrap-around combines both and transforms the whole ground floor, at a cost that starts to approach a small two-storey scheme.

Two-storey extensions are the value option once you need bedrooms as well as living space, because the expensive elements, foundations and roof, are already being paid for. On a standard semi a two-storey rear extension typically yields a kitchen-diner below and a double bedroom with ensuite above, and it usually needs a planning application where a single-storey version would not.

What the money actually buys

Where the budget goes on a typical 20m² rear extension
Element	Share of cost	Notes
Groundworks and foundations	15 to 25%	Depth is set by ground, trees and drains, not by choice
Shell: walls, roof, structure	25 to 30%	Steel beams where the rear wall opens up
Glazing and doors	10 to 20%	Bifolds, sliders and rooflights are the biggest optional swing
First fix services	8 to 12%	Electrics, plumbing, heating extension, drainage
Plaster, joinery, decoration	15 to 20%	Includes making good where old meets new
Floor finishes and fit-out	10 to 15%	Kitchen supply is usually a separate budget line

Two items reliably surprise people. The first is the cost of joining old to new: reforming the existing rear wall, threading heating and drainage through it and re-plastering the rooms it touches is real work that never appears in a headline square-metre rate. The second is glazing. The difference between a decent double-glazed slider and a wide aluminium bifold run with a structural head can be five figures on its own.

Ground conditions on Teesside

Most of the conurbation sits on glacial boulder clay, which is a reasonable bearing material but a shrinkable one. That matters where mature trees stand near the build line, because clay heave and shrinkage rules push foundation depths from a routine 900mm to 1.5m, 2m or deeper, or force a change to a piled and beam solution. On former industrial land around Grangetown, South Bank, Haverton Hill and parts of Hartlepool, made ground and old foundations can be present, and trial holes are cheaper than surprises.

Drainage is the other local variable. Older terraces frequently have shared or combined drainage running exactly where the new footprint wants to be. Building over or within three metres of a public sewer requires a build-over agreement from Northumbrian Water, which is a routine application but one that takes weeks, so it belongs at the start of a programme rather than the middle.

Permitted development or an application

Single-storey rear extensions of up to three metres on a semi or terrace, or four metres on a detached house, generally sit within permitted development, doubled under the prior approval route subject to a neighbour consultation. Two-storey work, anything on a principal elevation facing a highway, and anything in a conservation area normally needs a full householder application. Flats have no permitted development rights at all.

Whichever route applies, building regulations approval is required and is worth treating as the real technical test: structure, insulation, ventilation, fire escape, drainage and energy performance are all checked, and the completion certificate is what a future buyer's solicitor will ask to see.

The programme

A realistic single-storey extension runs 10 to 16 weeks on site. Weeks one and two are set-up, dig and foundations. Weeks three to six take the shell to roof level and get it watertight, which is the point at which the project starts to feel real. First fix, plaster and drying out fill the middle. The final fortnight is second fix, flooring, decoration and the break-through into the existing house, which is deliberately left as late as possible so you keep a sealed home for most of the job. Two storeys extends that to 16 to 24 weeks and brings a party wall notice into play where a boundary or shared wall is involved.

Getting a comparable quote

Extension quotes are only comparable when they price the same specification. Before asking for numbers it is worth fixing the glazing type, the floor finish, whether underfloor heating is included, who supplies the kitchen, and whether making good the existing rooms is in or out. Send the same information to whoever quotes and the spread between prices tells you something real. Send vague sketches and the cheapest quote will simply be the one that assumed the least.