Granny Annexe Cost UK 2026: What TRJ Actually Charges and What Changes the Price

If you have been researching the cost of a granny annexe in the UK, you have probably already discovered that getting a straight answer isn't easy.

One website says £60,000. Another says £80,000. Another advertises an annexe "from £50,000", while somebody else tells you to budget £150,000.

The confusing part is that all of those figures can potentially be correct — because they may be describing completely different buildings and, more importantly, completely different levels of completion.

At TRJ Construction, we build SIP granny annexes from the ground up. That means foundations, structure, insulation, roofing, windows and doors, electrics, plumbing, heating or air conditioning, kitchens, bathrooms and internal finishing.

So rather than giving you an attractive starting price that bears little resemblance to the final invoice, this guide explains what a properly completed granny annexe actually costs in 2026, where the money goes, and why two annexes of exactly the same floor area can have very different prices.

So, how much does a granny annexe cost in 2026?

As a realistic guide, TRJ granny annexes generally fall around £2,200–£2,500 per square metre, depending on design, specification, site conditions and services.

But there is something important we tell customers from the beginning:

Price per square metre is useful for estimating a project. It is not a reliable way of pricing the finished building.

What are you actually paying for?

When someone looks at a 40m² annexe and multiplies 40 by the price of a garden room, the resulting number can look very attractive.

Unfortunately, a granny annexe isn't simply a garden room with a bed in it.

It is effectively a small self-contained home.

That distinction accounts for a large proportion of the cost.

1. The ground underneath it

Before thinking about kitchens, flooring or bifold doors, we need to know what we're building on.

TRJ commonly uses screw-pile foundations, with piles typically around 1.25m deep depending on ground conditions and the engineering requirements of the project.

We like this system because it can dramatically reduce excavation and concrete compared with traditional foundations.

It also suits the way we construct SIP buildings: accurate, engineered and relatively fast.

But every garden is different.

Poor access, sloping ground, trees, existing structures, unsuitable soil or unusual structural requirements can all affect the foundation cost.

This is one of the reasons we are cautious when somebody asks:

"How much for a 40m² annexe?"

Before giving a final price, we want to know where those 40 square metres are going.

2. Getting drainage to the annexe

This is probably one of the most underestimated costs.

Your new bathroom might only be three metres from the bedroom.

But the relevant question isn't how far the toilet is from the bedroom.

It's:

How far is the annexe from the property's existing drainage system?

If we can achieve the correct fall, a conventional gravity connection may be relatively straightforward.

If we can't, a pumped drainage solution may be required.

We have solutions capable of moving waste considerable distances where the site requires it, but pumps, chambers, excavation, pipework and making good obviously affect the final price.

A beautiful kitchen gets noticed.

The 30 metres of drainage underneath the garden doesn't.

Unfortunately, you need both.

3. Electricity isn't just "a few sockets"

A self-contained annexe needs a properly designed electrical installation.

Depending on the project, that can include a dedicated consumer unit, RCBO protection, surge protection, lighting circuits, socket circuits, kitchen supplies, appliances, external lighting, data points and air-conditioning supplies.

Then we have to get an appropriately sized supply from the main property to the annexe.

If the annexe is five metres from the house, that's one situation.

If it's at the end of a 40-metre garden, it's another.

Cable size, route, trenching and existing electrical capacity all matter.

As a NAPIT-registered electrical contractor, this is an area we particularly don't believe in treating as an afterthought.

4. SIP construction costs more than a timber shed — for a reason

TRJ annexes use Structural Insulated Panels (SIPs).PU (FOAM INJECTED )

Our typical construction uses approximately:

125mm SIP external walls (low level building ) 150 mm SIP external walls for Granny Annexes
150mm SIP roof (low level building ) 175 mm SIP roof for Granny Annexes
150mm insulated SIP floor/base for all

A SIP isn't simply insulation stuffed between timber studs.

The structural skins and rigid insulated core work together as an engineered panel.

Once erected and correctly sealed, the result is a very well-insulated, airtight building envelope that can be assembled surprisingly quickly on site.

We have worked with SIP construction for many years, and for small independent residential buildings it remains our preferred method.

The panel package isn't necessarily the cheapest way to put four walls around somebody.

We don't think the cheapest four walls should be the objective when somebody is going to live inside them.

5. Windows and doors can change the price dramatically

Two annexes can have identical floor areas but thousands of pounds difference in glazing.

For example:

Annexe A might have standard uPVC French doors and three conventional windows.

Annexe B might have large aluminium sliding doors, floor-to-ceiling glazing and aluminium windows throughout.

They're both 40m².

They're not the same £40m².

Aluminium generally costs more than uPVC, but it gives slimmer profiles and can be particularly effective on contemporary designs.

We offer both because this is an area where personal preference genuinely matters.

6. Kitchens: £5,000 and £15,000 kitchens occupy roughly the same space

This is another reason online square-metre calculators can be misleading.

Both kitchens might contain:

a sink,
an oven,
a hob,
a fridge,
storage,
a worktop.

But cabinet quality, worktops, appliances, handles, splashbacks, lighting and installation can produce a huge difference in cost.

The same applies to bathrooms.

A standard shower enclosure and ceramic basin perform essentially the same functions as a large walk-in shower, concealed fittings, wall-hung WC and premium tiling.

We establish the specification with the customer rather than quietly building expensive allowances into every project.

7. Heating and cooling

A small, well-insulated SIP building doesn't necessarily require a complicated conventional heating system.

TRJ frequently installs LG air-conditioning systems, providing both heating and cooling.

That's particularly useful in a modern annexe because the same system that keeps the building comfortable in January can cool it during a hot July afternoon.

Again, specification matters.

One unit in an open-plan one-bedroom annexe is very different from designing climate control for a larger two-bedroom layout.

8. Exterior finish

This is where an annexe becomes part of the property rather than simply a building at the bottom of the garden.

Depending on the design, we use finishes including render, composite cladding, Thermowood and other contemporary materials.

You can spend surprisingly different amounts covering exactly the same wall.

We normally help customers decide where spending more will genuinely improve the finished building and where it won't.

That's an important part of building to a budget.

9. Planning, design and Building Regulations

The physical building isn't necessarily the entire project cost.

The appropriate planning route depends on the proposed annexe, its use, design, position and relationship with the main dwelling.

There can also be costs associated with drawings, structural calculations, applications, surveys and Building Control.

We would rather identify these at the beginning than advertise an artificially low building price and introduce them later.

10. Access to your garden matters more than most people realise

Here's something you rarely see in online annexe calculators.

Can we actually get the materials into your garden?

If there is good side access, construction is easier.

If everything has to travel through a narrow passage, through a garage or in some cases through the house, the same building requires considerably more handling.

Then consider:

Where can deliveries unload?

Where can materials be stored?

Can a skip be positioned nearby?

How far do trades have to carry equipment?

Can machinery access the site?

Those aren't exciting questions when you're choosing your new kitchen.

They're very important questions when we're calculating what the project will actually cost.

What about the £50,000 granny annexes advertised online?

They exist.

But always ask one question:

"What exactly is included?"

A published UK survey of 176 advertised annexe prices in 2026 found starting prices ranging from approximately £52,000 for compact studios to £155,000+ for larger two-bedroom annexes. It also found that many advertised prices exclude items such as groundworks and mains connections, which can add substantial amounts to the finished project.

That's why comparing two headline prices can be meaningless.

One quotation might include:

• foundation
  SIP structure
  roof
  windows and doors
  full electrical installation
  plumbing
  drainage connection
  bathroom
  kitchen
  flooring
  decoration
  heating/cooling
  external finish
  certification

while another £20,000-cheaper quotation may exclude five of them.

The cheaper quotation isn't necessarily cheaper once both buildings are finished.

Does a bigger annexe always cost proportionally more?

Interestingly, no.

Suppose we build a 30m² one-bedroom annexe and a 50m² two-bedroom annexe.

The larger building has more floor, roof and wall area.

But both may still require:

one kitchen,
one bathroom,
one consumer unit,
one hot-water system,
one drainage connection,
one electrical supply,
one set of professional/design costs.

Those fixed costs are spread across more square metres in the larger building.

Consequently, smaller annexes can sometimes have a higher cost per square metre.

This is something a simple online calculator often misses.

Where can you save money without making the annexe cheap?

This is a conversation we regularly have with customers.

If a project is over budget, our first reaction isn't:

"Use less insulation."

There are much better places to adjust the specification.

For example, choosing good-quality uPVC instead of aluminium can save money.

Simplifying the shape of the building can save money.

Reducing very large areas of glazing can save money.

Choosing sensible bathroom fittings rather than designer products can save money.

Selecting a good laminate or LVT floor rather than a premium flooring system can save money.

A straightforward kitchen can save thousands without making the building unpleasant to live in.

What we don't like compromising on is the part you won't easily replace later:

foundation, structure, insulation, waterproofing and electrical safety.

Changing a kitchen worktop five years later is easy.

Correcting a poorly constructed building envelope isn't.

Why TRJ uses SIPs for granny annexes

For us, speed is useful — but it isn't the main reason.

A SIP structure allows much of the building envelope to be manufactured accurately before arriving on site.

Once the foundations and base are prepared, the structural shell can progress very quickly.

That means fewer weeks of traditional wet construction in the garden and considerably less disruption than many customers expect when they first hear the word "building project".

We also use battery-powered equipment wherever practical and try to keep heavy site activity to a minimum.

For somebody building an annexe for an elderly parent while continuing to live in the main house, less disruption matters.

What will make your particular annexe more expensive?

If you're trying to establish a budget before speaking to a builder, these are the things I'd look at first:

Size and shape — simple rectangles are generally more economical than complicated footprints.

Ground conditions — difficult ground changes foundation requirements.

Distance from the house — particularly important for electricity, water and drainage.

Access — difficult access means additional labour and handling.

Glazing — large aluminium systems can substantially increase the price.

Kitchen specification — one of the easiest places to add or remove thousands of pounds.

Bathroom specification — especially extensive tiling and premium fittings.

Exterior finish — render, composite and timber finishes have different costs.

Heating/cooling — depends on layout and requirements.

Planning/design requirements — every property isn't identical.

Accessibility requirements — wider doors, level thresholds, accessible bathrooms and specialist fittings may change the design and specification.

What we would budget in 2026

If someone phoned TRJ today and said:

"I want a proper self-contained granny annexe, not a glorified garden room. What should I realistically budget?"

Our answer would be:

Start the conversation around £70,000 for our most compact one-bedroom solution and expect larger, higher-specification one- and two-bedroom annexes to move into the £120,000–£140,000+ range.

Then let us look at the actual garden.

Because the difference between a £90,000 project and a £140,000 project isn't necessarily somebody making £50,000 more profit.

It can be a completely different building.

One final piece of advice from a builder

Don't choose your annexe builder from the first number on Google.

And don't choose us simply because of a number on this page either.

Get a proper specification.

Put the quotations next to each other.

Then compare:

What foundation am I getting?
What are the walls made from?
What insulation am I getting?
Who is doing the electrics?
Is drainage included?
Are the kitchen and bathroom included?
What windows and doors are specified?
Is heating included?
Are flooring and decoration included?
What happens if access is difficult?
What warranties and certificates will I receive?

If two companies are offering genuinely equivalent buildings and one is £25,000 cheaper, that's worth investigating.

But first make certain they really are equivalent buildings.

A granny annexe isn't something most families buy twice.

It may become a parent's home, give an adult child independence, provide accessible accommodation or allow several generations of a family to remain close to each other.

We think it deserves to be priced — and built — accordingly.