How much more does a new build basement cost in the UK? Assuming you are building a house or extension above anyway.

I have two methods to produce a budget figure. Itemised and All-in.

I find, generally, that labour is just a little bit more than the other costs added together.

If you add up all the other expected costs, multiply them by 2.2 to get your total. Less if you plan to work yourself.

ITEMISED. You might like to prepare your own spreadsheet with

Soil investigation

Structural engineer

Excavator and operator

Muckaway

Blinding

Steel reinforcement for the floor slab

Floor slab concrete

Timber

Steel reinforcement for the walls

Wall concrete

Sundries

Labour

Extras for difficulties

Contingencies

Your basement only needs to fulfill one of these criteria to be a good idea.

The basement needs to cost less than the value it adds, and I need to be able to afford it.

The basement needs to fulfill wishes. It doesn't need to add as much value as it will cost if it provides what I want that is not available to me any other way.

Soil investigation

I have a page about soil investigations, and at the bottom a dozen or so phone numbers. Prices vary hugely. I would approach at least half a dozen for prices. Very roughly £2,000 for a good investigation and report.
basement construction soil investigation

Structural engineer

Again, prices vary wildly. But the main issue is finding one who will put you on his waiting list. Your architect might recommend an engineer who, it turns out, is great at complex roofs but who never designed a domestic basement before. Beware the engineer who copies and pastes a design that was for underground car parking beneath an 8 storey office building. Yes. It frequently happens. If you are paying me in any way I will put a firework up their backside and make them do the work properly.

Excavator and operator

I prefer finding a local hire company who will supply you with the operator as well. That way, the theft insurance is down to them not you, a new build and they should zero rate VAT, they worry about fuel, fuel bowser and bund and so on. You need to be organised so that they can dig without being held up and you can continue without leaving the excavation open to the weather for any length of time.

Muckaway

Muckaway varies in price because the distance they travel to tip each load varies. Also, a lorry with a grab costs more than a plain tipper lorry and it carries less each load because the weight of the grab is part of their maximum weight allowed on the road. A bigger excavator, 13 tonnes usually, costs more but saves money because it can fully load cheaper lorries.

Blinding

Allow for 75mm average depth of blinding concrete. Blinding needs to protect the excavation from the weather and support the stools/chairs/blocks that hold the steel reinforcement clear for concrete to get underneath. It should be stronger than the soil you remove, not super-strong. You can have it very, very wet and spread it around with rakes or very dry and spread it with the excavator bucket.

Steel reinforcement for the floor slab

Many structural designs will want two layers of A393 mesh and two faces of 16mm starter bars. But some won't. Mesh 15kgs/m² of floor area. Starter bars 40kgs per metre of perimeter. Check the useful-info page for supplier phone numbers.

Floor slab concrete

Cement more than doubled in price. PFA and GGBS got in short supply. Aggregates travel by road and diesel went up as did driver rates. Most floor slabs are 300mm thick. Concrete might cost you £160/m³ for the correct too-stiff-but-otherwise-waterproof mix of concrete. My admixture is the best and the cheapest, because I have no overheads, at £39/m³.

Timber

The more you buy the further up the wholesale chain you can buy from. The secret is for your formwork to be mainly built with what you can re-use in your new home. Try to get a price from everyone. Whole packs will be a cheaper rate than loose boards. If the wholesaler at the docks won't give you a price you might have to buy from a local merchant. Timber merchants include delivery and they might take back packs unopened whereas you may need to find a lorry to collect from the docks.

You can, hopefully, download this very simple spreadsheet formwork timber.xlsx. If it won't download, email and ask me to send it to you. No charge just for this. No guarantees. It is for budget purposes.

Steel reinforcement for the walls

Very often two faces of A393 and not much more. About 55kgs per linear metre.

Wall concrete

Very often 300mm thick. £160/m³ for concrete and £39/m³ for my admixture.

Sundries

Calculate every element based upon your structure size and my suggestions. Add 5%.

Labour

Having recalculated a total. Add 120% for labour. A lot less if you are going to do some/most/all the work yourself.

Extras for difficulties

There is another numbered list just below here with some hints to make you think whether there are any other costs you should include. You might not expect other costs until you get your soil investigation report back or you realise you have a Party Wall Act issue.

Contingencies

In the vast majority of cases, after you have your THOROUGH-no-corners-cut-soil-investigation-report and you know you will or won't need Party Wall Surveyors, piling and so on you should hopefully not need any contingency fund - but it would be foolish not to have access to more money, because you don't want to have to abandon your project mid-way. Loose sand and/or water are the usual issues that people crossed their fingers about but got caught out anyway. Sand or silt can be just a thin vein but will cause problems if unexpected water washes it out.

All-in budget pricing.

For several years until Brexit and Covid then inflation put prices up, I had a very simple yet reliable formula.

Add together the outside area of the floor slab and the outside area of the retaining walls.

If your basement is to be 12m x 8m externally, that would mean 12 x 8 = 96. 12 + 12 + 8 + 8 = 40. 40 x 3m high = 120.

96 + 120=216 sq m.

It used to be that £200 for each of 216 sq m is what I would expect my clients to be able to build their basement for if they paid all their labour themselves and all the kit and all the materials and employed me to manage throughout.

I reckoned that was 30% less than an experienced contractor who would want £285 for each square metre.

But some clients did so much of the work themselves, and bought second-hand timber, got a mate with a digger to excavate for free, and so on, one reckoned his basement cost him £90 a square metre.

But the question is, what figure should you use today?

I'm thinking £420.

Pumps are nearly twice, concrete is nearly twice, decent labour has probably doubled (but A-level students might still be £50 a day and they are better than cheap labourers who want 100), and so on.

It might be most reliable to get a price from an experienced contractor then assume I can still save you 30% or you could save even more doing it yourself.

And, don't forget. If you use an experienced contractor he will leave you a lot of leaks to repair - which could be impossible and you will need to buy internal drainage.

If I work with you to build my way I guarantee you that your basement will always be dry from the reinforced concrete alone. No sump, no pump, just dry.

A bit more detail about less usual costs.

1.

Your access might be poor. Removing all the soil by wheelbarrow through a mid-terrace house and bringing all the steel and concrete in the same way is many, many, many, many times more expensive than a 13 tonne excavator digging the hole and loading trucks at its side. I would say stop dreaming now if the only access is through your home.

Otherwise there might be some minor access issue that increases cost slightly.

In this example, a site I visited but heard no more afterwards, he dug his hole with a mini digger, took all the soil out in a motorised wheelbarrow he walked behind, tipped it all in his tiny front garden and it was taken away by grab lorry.

You can tell that no truck could get between the house and the garage.

This means tonnes of steel will need to be carried in and all his concrete will have to be pumped through a line laying on the ground.

A line pump easily doubles the work pouring the floor because you have to keep dragging a pipe around your floor that takes 4 men to manipulate when full of concrete.

This same issue, manipulating a full pipe, becomes a lot worse when you are trying to fill the top of the wall, though you might put a foor in at top of wall level and shovel the last few feet.

new build basement cost

2.

The water table can mean the difference between digging almost to the water cheaply, and digging into the water at 20 times the cost. In cities you sometimes see a Victorian house with some steps down to the half-depth basement and some steps up to the slightly elevated ground floor. I always think that the developers dug as far as they could before costs went up because of the water table, and they started the basement just above.

Digging into the water table should not be confused with digging an excavation and water gets in and needs to be pumped out periodically.

The difference is soil stability.

If you dig into sand with water running through it the sand wants to be washed in by the water that prefers your hole to struggling through the ground. This was in Twickenham where the gain in house value was greater than tens of thousands of pounds to overcome River Thames gravel and tide water.

If you are dealing only with some water after it rains, I expect that you would find a lay-flat discharge hose a time-consuming pain in the butt to keep going. It can be worth setting up a rigid and strong discharge pipe.

3.

The Party Wall Act could make it difficult to be allowed to build a basement. Particularly if an affordable method cannot be found to excavate without damaging neighbouring properties. Many basements get caught up in the Party Wall Act because the excavation or piles (not just the basement structure) will be within 3m of a neighbouring building.

This is a very common solution.

You will have to pay for your neighbour's and your own party wall surveyor and expect them to take months to come to an agreement. You ought to have your detailed soil investigation report and its party wall recommendations first, or else everyone is guessing and surveyors do not sign off on guesswork.
You can view and print a government guide to the Party Wall Act here.

4.

Your dream might be too ambitious.

Not many swimming pools go ahead because the additional space and cost for all the pool equipment gets too expensive. A pool 10m x 4m is to be built independently inside this basement with the kit along the far side. Then a steel frame orangery over it.

I would really ask yourselves if you will truly use a swimming pool very much. I have visited people who thought their grandchildren would often visit, but they didn't and their pool was rarely used. Plus, they quickly grow up.

Another issue is do you build it long and thin to swim or square to play? Long enough and square enough as well becomes very big.

One lady I know has a pool in her house but she goes to the leisure centre to swim instead because there she meets friends.

How long before you finish? This pool, below, was already 8 years old when I was asked to fix leaks through the concrete.

new build basement cost

I would have a factory built spa pool that can be still, have a current to swim against and be a jacuzzi. This image is from endlesspools.co.uk.

new build basement cost

Foul drainage in a basement sometimes increases cost too much.

Especially if a sump for a pump goes deeper into the water table or going deeper invokes the party wall act or going deeper creates difficulties of soil stability and introduces the cost of steel sheet piles.

These people destroyed their fibreglass sump with soil and water crashing in while they tried to get it into position. They abandoned it and built a concrete sump effectively in water, which was not easy. The second image is a concrete sump I built in dry, stable ground.

I don't think big sumps for foul drainage are a good idea at all. If you have a guest who uses a dozen wet wipes, you could be clearing a dozen pooey blockages before it all works again.

I prefer Saniflow-type macerators.

The first guest wet wipe would block it but it is easier to get to and you immediately educate your guest not to use a second.

5.

Planning to park cars in a basement is expensive because you either need an expensive car lift or a lot of space for a ramp.

This is from Google maps. I built the basement for this below ground garage with games room above in 2012. You can see that the car ramp required a lot of space.

A basement is probably cheapest, relatively speaking, if it is going to be beneath a new house that is going to be built on top more than 5m from a neighbour and there is enough space (and no water or unstable ground) to batter the sides of the excavation and still get the excavator and the muckaway trucks off the road.

A basement is probably most cost-effective if the basement footprint is the same as the footprint of the house, or new house extension, above.

But making the basement bigger than the house and laying patio on top of the extra footprint can be good value as well. Budget £175/m² for the waterproof concrete roof then ask me how you can build it for that.

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