Extending Out at the Back or Side of Your Home

Fix Your House designs and builds house extensions across Barnet, Edgware, Mill Hill, Hendon and Finchley — single-storey rear extensions, side returns, wrap-arounds, double-storey extensions and garage conversions.

Our founders lead the design themselves: Mohammad Reza Hosseini is our Founder and Niloofar Tootoonchi is our Head of Interior Design. On an extension that matters more than it sounds. Most extensions are designed by someone who never sees the site again, and built by someone who inherits the drawings. We do the design and the build, so when something is discovered in the ground the person redesigning around it is the person who drew it in the first place.

Extensions are quoted after a survey. We do not publish a fixed price for them — see why below. Call 07777 759333, answered 24/7, for a free quote.

What We Build

Single-Storey Rear Extensions

The most common project in this part of London. Typically a new kitchen and dining space across the back of the house, often with bi-folds or sliding doors and roof lights.

Side Return Extensions

Filling in the narrow alley beside the rear addition of a Victorian or Edwardian terrace. Small in footprint, disproportionately large in effect, because it turns a corridor kitchen into a room.

Wrap-Around Extensions

Rear and side return combined. More structural work, and usually the point at which planning permission rather than permitted development becomes the route.

Double-Storey Extensions

Adding a bedroom or bathroom above a ground-floor extension. Foundations, structure and party wall considerations are all more involved.

Garage Conversions and Infill

Converting an attached garage into habitable space. Common in 1930s semis around Mill Hill and Edgware, and often the cheapest square metres you will ever add.

Rear Extension and Full Refurbishment Together

Most extension clients end up refurbishing the rest of the house at the same time. Doing both under one contract avoids the standard problem of a new extension making the original house look worse. There is a section further down on how that job actually runs.

Weighing up building out against converting up? On many North West London houses the roof space can add a bedroom without losing any garden, and the planning route is often simpler. See our page on loft conversions in North West London, which covers how to check your head height and when permitted development applies.

Planning and Building Regulations

This is the part that catches people out, so here it is plainly.

Permitted Development

Many single-storey rear extensions can be built under permitted development rather than a full planning application, subject to limits on depth, height, materials and how much of the garden is covered. Beyond the standard limits, the Larger Home Extension prior approval route allows deeper single-storey rear extensions, subject to a neighbour consultation process run by the council.

When Permitted Development Does Not Apply

• Flats and maisonettes have no permitted development rights for extensions.
• Conservation areas restrict what can be done, and Barnet has a number of them.
• An Article 4 direction can remove permitted development rights on specific streets.
• Listed buildings need listed building consent regardless.
• Rights may already have been used up or removed by a condition on an earlier permission.

We check the planning history and the designation for your specific address before advising, rather than assuming.

Building Regulations

Building Regulations approval is required for an extension whether or not planning permission is needed. They are two separate systems. Approval covers structure, insulation and thermal performance, fire safety and escape, ventilation, drainage and electrical safety under Part P.

Party Wall etc. Act 1996

If you are building on or against a boundary, cutting into a shared wall, or excavating within three metres of a neighbour’s foundations, you must serve notice on your neighbours under the Party Wall etc. Act. On a terrace or semi — which is most of Barnet — this applies more often than not. Notice periods are one or two months depending on the type of work, so this needs to start early.

Drainage and Other Consents

Building over or near a public sewer needs a build-over agreement with Thames Water. Extensions over 100 square metres of new floorspace can attract the Community Infrastructure Levy, although a residential extension exemption may be available if it is claimed before work starts.

We coordinate all of this for you. Planning applications, Building Control submissions, structural engineering, party wall notices and any drainage consents are arranged and programmed as part of the project, using qualified specialists where the work has to be signed off by one. You deal with us rather than chasing each consultant separately.

Extensions on North West London Houses

Victorian and Edwardian Terraces — Finchley and Hendon

Shallow foundations, solid brick walls and a narrow side return. Underpinning next to the party wall is often needed, and rear access is usually through the house, which affects both the programme and the price. The side return infill is the classic move here.

1930s Semis — Mill Hill, Edgware, Burnt Oak, Hendon

Generally deeper gardens and side access, which makes these the easiest houses in the area to extend. The usual project is a rear extension opening the kitchen into the garden, frequently combined with a garage conversion.

Post-War Housing

Often fewer structural surprises, but check for estate-wide planning conditions that removed permitted development rights.

Rear Extension and Full Refurbishment as One Job

A lot of people search for exactly that phrase, and it describes what we are usually asked for more accurately than the word “extension” on its own. You want the back of the house opened up, and you want the rest of it — bathroom, wiring, heating pipework, floors, decoration — brought up to the same standard, so that the new part does not make the old part look tired.

Why It Is Better Under One Contract

Split the work between an extension builder and a separate refurbishment team and you create a seam. The seam is where the money goes. The new floor is a few millimetres out, so the flooring contractor will not lay onto it, and the builder has finished and left. The consumer unit needs upgrading for the new circuits, but that was in nobody’s scope. The plasterer waits three weeks because the first fix was priced by someone who was never told the rest of the house was being rewired. None of these are difficult problems. They are only expensive when there is nobody whose job it is to own them.

Under one contract there is one programme, one set of drawings and one company answerable for the joins between old and new.

How the Sequencing Works

Done properly the two halves are not run side by side. They are interleaved, and the order matters.

1. Design both at once. The extension layout and the refurbished layout are drawn together, so the kitchen is not designed twice and the soil stack is not moved twice. We do this in-house and produce 3D visualisations and BIM models, so you can see the finished house before anything is ordered.
2. Build the shell first. Groundworks, structure, roof, windows and doors. The existing house stays broadly liveable while this happens, because the new build is outside it.
3. Break through last. The opening between old and new is formed once the extension is weathertight. That single day is what turns two buildings into one house, and it is the point after which the original rooms become a building site too.
4. Strip out and first fix across the whole house. Electrics, plumbing and heating pipework are run as one system rather than a new system bolted onto an old one. This is the step that is close to impossible to get right when the contracts are split.
5. Plaster, second fix and finishes throughout. Floors, joinery, decoration and finishes run continuously from the new space into the old. It is the only way to get the levels, the floor finish and the paint to actually match.

What It Means for Living Arrangements

Be honest with yourself about the middle of the job. Up to the break-through most people can stay. After it, with the whole house on first fix, most people move out for a period. We will tell you at quote stage roughly where that window falls, because planning it late is how people end up paying for a rental they did not budget for.

Quoting a Combined Job

We quote a rear extension and full refurbishment as one specification with the two halves priced separately inside it. You can see what the extension costs and what the refurbishment costs, and defer or drop parts of the refurbishment without unpicking the whole price. Anything genuinely unknowable — ground conditions, or what is behind the plaster in a Victorian terrace — is carried as a named provisional sum rather than buried in the rate.

How We Work

1. Free site visit. We look at the house, the garden, the access and the boundaries, and talk about what you actually want the space to do.
2. Feasibility and design. What is achievable under permitted development versus what needs a planning application, with layout options.
3. Consents. Planning or prior approval, structural calculations, Building Regulations, party wall notices and any build-over agreement.
4. Fixed written quote. A full specification with labour and materials separated, plus a named provisional sum for anything genuinely unknowable, such as ground conditions.
5. Build. Groundworks, structure, roof, weathertight shell, first fix, plaster, second fix, finishes.
6. Completion. Building Control final certificate, electrical certification from the registered electrician who carried out the notifiable electrical work, and a snagging walk-through before the final payment.

How Much Does an Extension Cost?

We do not publish a price for extension work, and we would rather explain why than hand you a number that will not survive the survey.

An extension is not a product with a shelf price, and margins in this part of the trade are thin. Discounting below them does not produce a keen quote — it produces a builder who runs out of money halfway through your project and starts looking for variations. You have probably heard that story from someone.

So on extensions we quote after a survey, and we compete on the design, the specification and the fact that we do not disappear. Where we can find a genuine saving, we pass it on and show you where it came from.

What Drives the Figure

What an extension costs depends on ground conditions, access, glazing, roof type and specification, and on whether structural steel and Building Control work are needed. Two extensions of the same floor area on the same street can come out very differently for those reasons, which is why we survey first and put the figure in writing.

What a Quote From Us Includes

• An itemised specification, with labour and materials separated
• Named provisional sums for anything that cannot be known before the ground is opened
• A written programme with a start date and a finish date
• A stage payment schedule tied to completed work, not to dates
• Fees for planning, Building Control, structural calculations and party wall matters shown separately
• A final figure that is the figure you pay — no VAT added on top

Frequently Asked Questions

How much does a house extension cost in North West London?

We quote after a site visit rather than publishing a rate. Where your project lands depends on ground conditions, access, glazing and specification. The site visit and the quote are free.

Why do you not publish a fixed price for extensions?

Because margins in this part of the trade are thin, and a headline discount on a job of this size would mean either cutting the specification or running out of money before completion. We would rather be straight about that than win the enquiry and disappoint you later.

Do I need planning permission for a rear extension?

Not always. Many single-storey rear extensions fall under permitted development, and deeper ones may be possible through the Larger Home Extension prior approval route, which involves a neighbour consultation. Permitted development does not apply to flats, and is restricted in conservation areas, on listed buildings, and where an Article 4 direction or a planning condition has removed it. We check your specific address before advising.

How long does an extension take to build?

Getting consents typically takes two to four months before anyone digs, depending on the route. A single-storey rear extension then usually takes three to five months on site. A double-storey extension or a wrap-around takes longer. You get a written programme with your quote.

Do I need a party wall agreement?

If you are building on or against the boundary, cutting into a shared wall, or excavating within three metres of a neighbour’s foundations, yes. On a terrace or a semi that is usually the case. Notice periods are one or two months, so it needs to be started early rather than treated as paperwork at the end.

Can you do the design as well as the build?

Yes, and it is how we prefer to work. Both founders come from an interior design background, so the design, the drawings and the build stay with one company.

What areas do you cover?

Barnet, Edgware, Mill Hill, Hendon, Finchley and the surrounding North West London area.

Can I live in the house while the extension is being built?

Usually yes for a single-storey rear extension, because most of the work happens outside the existing external wall until quite late in the programme. The disruption concentrates at two points: the groundworks at the start, with noise, spoil and heavy traffic around or through the house, and the break-through at the end, when the existing rear wall is opened up and the house is briefly open to the weather and full of dust. Between those two, life is more normal than people expect. It gets harder if the extension takes in the kitchen, since you may be without one for a stretch, and harder again on a two-storey extension, where scaffolding, roof work and first-floor structural work reach the bedrooms. Say at design stage whether you intend to stay in the house, because the sequence of work can be planned quite differently for an occupied property than for an empty one.

What happens if something unexpected turns up in the ground?

You are told and it is repriced before it is built, not afterwards. Ground is the least predictable part of any extension, which is why a foundation figure is a considered allowance rather than a fixed price. Locally the usual cause is clay: much of North West London sits on London Clay, which shrinks and swells with moisture, and where there are mature street trees or a large garden tree nearby, Building Control will normally require foundations deeper than the base case, sometimes considerably deeper. The other frequent surprise is drainage, because old clay drain runs and shared sewers often cross exactly the footprint people want to build on, and building over or near a public sewer requires a build over agreement with Thames Water. Both are checked as early as possible rather than left to the dig, but neither can be fully known until the ground is open.

Do I need Building Control approval as well as planning permission?

Yes, and they are two separate things that are commonly confused. Planning permission is about whether you are allowed to build it at all: size, height, position, appearance and effect on neighbours. Many rear extensions need no planning application because they fall under permitted development, though that is easier to rely on for a house than for a flat, and permitted development rights can be removed in a conservation area or by a condition attached to the property earlier. Building Control is a different process about whether what you build is safe and compliant: foundations, structure, insulation, fire safety, drainage and ventilation. It applies to essentially every extension, including one that needs no planning permission whatsoever. A party wall agreement is a third separate matter again, and is between you and your neighbour rather than with the council.

Get a Free Extension Quote

Call 07777 759333 — answered 24/7 — or use our contact page. We will visit, look at what is possible, and give you an itemised written quote at no cost and with no obligation.

Related services: kitchen renovation, flooring, carpentry, plastering and tiling, handyman services, bathroom renovation, and our full service list.