It’s tempting to answer that question with a question: ‘How long is a piece of string?’. But actually, unless you have a unique and innovative architectural showpiece in mind, it is possible to estimate how much an extension costs with some level of accuracy.

Breaking down the ballpark costs at the start means you’ll know what kind of market you’re in when it comes to the interior fittings and help you to prioritise how your budget is spent. There’s no getting around the fact that we’re talking double figures here, whatever kind of extension you have. And that’s scary.

But the good news is that there are plenty of ways you can trim the costs and still have a great quality addition to your space. Here are the answers to all the big questions…

The cost per square metre of an extension will fluctuate depending on the property market in your area. So even extension ideas for small houses can prove costly depending on where you live. Architect and director of DesignFor-Me (an architect-finding service) Emily Barnes says ‘The rule of thumb is now £1,500 per square metre for a basic to mid-range extension. In London and the Southeast, this rises to £1,800-2,000 per square metre.’ So, if you are outside London/Southeast, according to DesignFor-Me:

A basic, 4m x 5m single-storey extension. This covers build costs only and none of the extras and interior fittings (see below).

A small 3m x 6m two-storey extension (build costs only)/

A small 3m x 6m two-storey extension with architect-designed cladding, bespoke glazing and a designer kitchen.

In London and the Southeast, you’ll pay more than the rest of the UK, due to higher labour and material costs. Robert Wood, MD of London-based construction company Simply Extend says, ‘Including VAT, a single-storey side return extension starts at £44k, a rear extension starts at £55-60k and a wraparound extension, which is a combination of a rear and side extension, connecting at the corner, costs from £65k.’

Try Homebuilding & Renovating’s extension cost calculator to get an estimate of your project.

Architects’ fees scale according to the total budget of the project. According to an independent survey by research company The Fees Bureau average charges break down as follows: £25k project, 10.7%; £50k project, 9.9%; £75k project, 9.5%; £150k project, 8.7%. 

Your final bill will also vary depending on the location, the complexity of the project and the level of service you opted for. The architect will usually put forward an itemised proposal breaking down the different stages of the work and their percentage of the total fee. This is to help you budget, but also makes it easier to decide if you want to opt out of parts and source them differently.

In simple terms, the stages are: Concept and design, technical plans for approval, and construction management. So, you could work up your own simple concept and even use free online software to create a design plan, then hand over to the architect for full tech specs with structural calculations, building regs and project management. Or, you could use them for the initial stages, then hand over to your builder for the rest. 

In fact, you don’t HAVE to hire an architect at all. There are other routes: for example, if you know what sort of design you want, you could hire a structural engineer instead. According to myjobquote.co.uk, this will cost between £400 and £4,000, depending on the complexity of the project.

You could use a design-and-build company if your project is simple, or commission a firm of architectural technicians to supply technical plans. There’s a whole raft of these online, offering packages for just a few hundred pounds.

However, bear in mind that if your extension is big or in any way out of the ordinary, an architect can save you money by giving you the best ideas for the space you have and controlling costs from start to finish.

At the very minimum, you’ll also have to stump up for…

Don’t forget to factor in the cost of fittings and furniture for your new space

Don’t get caught out – check all quotes as they’re likely to be exclusive of VAT.

Any new space is going to need power. Plus, brace yourself for the possibility that your electrician will find a problem with the existing wiring that needs sorting.

You’ll have kitchen cabinetry and appliances (average £10k), a new bathroom suite (average 5k), or new furniture to buy. Plus heating (underfloor or rads), flooring and lighting. 

If you’re adding a bathroom, also factor in…

If you can’t live in the house while the work is done, you’ll also need to budget for removal and renting elsewhere as well as updating your insurance policy to cover your property during the work. 

Related: Kitchen extension ideas – to maximise the potential of your extended space

There are lots of ways to shave costs down – here are 16 for starters. Some will be doable for you; others won’t:

Can you get the extra space you need with a garden building, garage conversion instead or conservatory instead? These are all considerably cheaper options and usually fall under Permitted Development Rights, so you won’t have to get planning permission.

Could your space needs be served by a conservatory instead?

Chris Thompson, a keen DIYer, tackled a two-storey addition to his period cottage himself and chronicled the experience in his blog, www.diyhomeextension.co.uk .

‘The obvious pro is financial,’ he says. ‘General estimates are that 80% of a builder’s quote is to cover labour costs, so you’re saving that. However, the con is that it will take longer. If you’re realistic about timescales and know what you’re getting into, that doesn’t have to be a problem, though. Builders have to work within a timeframe to make money; you don’t. You’ll be able to do things exactly as you want them done and have pride in the result. I know mine is done properly.’

A clear site makes for easier groundworks at the start. When planning the footprint of your extension, you’ll save money if there are no trees to take out or drainage to move. You could also save on labour costs if you remove shrubs and paving yourself.

We’d all love an architecturally interesting glass-and-steel edifice, but unfussy extensions with 90-degree angles rather than clever curves and slants will be quicker – and therefore cheaper – to build. Elegant curves cost more than simple right angles

It’s important that your extension is in proportion to your property anyway, but the smaller the job, the quicker it’ll go up.

Making sure every detail is spot-on before ground is broken means you’ll avoid changes of plan as you go. For water and sewage pipes particularly, this can be expensive and cause frustrating delays.

Timing the groundworks, build and roofing to take place during the summer months will reduce the chances of your project being paused due to heavy rain, wind, snow or frost. 

Choose a builder who will give you a fixed price and work to an itemised list of costs agreed up front, to avoid unexpected extras as the work progresses.

A great builder or architect will be able to do quality work that meets building regulations first time, control costs and be alert to ways to save you money along the way.

Usually, the building firm or architect you employ will provide the services of a project manager, but if you have the time and organisational skills, you could DIY. It would involve liaising with the architect (if you’re using one), working with the local authority’s planning and building control officers, finding tradespeople, directing the schedule of work and sourcing materials.

Timber-framed extensions cost less, even though the materials are more expensive, because they are quicker to build so give lower labour costs.

Tim Bromley, Federation of Master Builders member and MD of Wolfe Design and Build, says: ‘For larger extensions, timber frame can be a quicker option. It doesn’t suit every project, however. Traditional block and brick methods offer greater flexibility when attaching an extension to irregular profiles, such as period properties.’

Building an oak-framed extension can be cheaper than bricks and mortar

Insulating Concrete Formwork’ blocks are essentially polystyrene Lego bricks that fit together to create a double wall that’s filled in the middle with poured concrete. It’s a quick, low-skilled job for reduced labour costs. It’s also great for heat and sound insulation.

Tim Bromley, a member of the Federation of Master Builders and MD of Wolfe Design and Build comments, ‘The key advantages of ICF blocks are build time and thermal efficiency for large extensions, but the thickness of ICF walls does reduce the internal space. Maximising space is often key in this kind of project, so that’s an important consideration.’

Dressing the block work structure with shiplap cladding or simple render works out cheaper than skinning it with an outer layer of bricks. Expert Tim Bromley confirms, ‘The most cost-effective method for a block-work extension is exterior render, including those with a timber frame.’

Finishing an extension with render rather than brick facing can reduce costs.

Image credit: Tibor Silva/Simply Extend

When it comes to arranging the interior of your new space, leave soil stacks and pipework where they are – even if it means compromising your dream layout – to avoid spending on relocating them.

Bespoke glazing is budget-busting, but you can still achieve a light, airy space with Velux roof lights and ready-made windows and doors. If you’ve got your heart set on bifolds, expect to pay around £2,750 for a 2.7m aluminium frame unit or £1,650-2,000 for UPVC.

Choose ‘ready to assemble’ units and fit them yourself if you’re a competent DIYer to save on the additional installation costs. Sliding, rather than folding, aluminium-frame doors tend to be 25% more expensive. Velux centre-pivot roof windows are a cheaper option to bespoke glazing.

Fitted joinery and tiling are beautiful, but labour-intensive. Paint walls rather than have them tiled, choose carpet, LVT (luxury vinyl tile) or engineered wood flooring, and find off-the-peg cabinetry you love.

Painted walls and off-the-peg cabinetry will bring your project in on budget.

Robert Wood, Managing Director of London-based construction company Simply Extend says, ‘It is in general cheaper to do a loft conversion due to the fact that no foundations are needed as you are building on top of an existing structure.’

Most loft conversions also don’t need planning permission. However, if your loft conversion would involve changing the roof structure, costs will go up.

There’s not much new research on this, but a 2017 Home Improvement Index survey by Zopa of 1,550 homeowners revealed that an extension added an average of 16% to the property value and gave a 57% return on investment. 

An older report – Nationwide’s House Price Index from 2014 – found that extending out or up added 21% to the property value, or 5% for every 10% increase of floor area.

Obviously, the added value will depend largely on where you live and the type of space that’s in demand, so it’s worth researching the sold prices of houses in your area and looking at what improvements they’ve had. Unless you’re in your ‘forever’ home, it pays to think strategically and add larger kitchens and extra bathrooms in family homes, or a garage in areas with restricted parking.