Is Basement a Good Idea for New Self Build ? | Building on Sloping Land Plots

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The Essential Guide » Design and Finance » Is a Basement a Good Idea in New Self Build Design | Building on Sloping Land)

Is a basement a good idea in a new design? (Including: “Building on sloping sites”)

Answer? - Not that often in the UK:

The chances are that generally in the UK, a “fully fledged” basement, like the ones we see in the American movies, will not be a good idea, nor will it be an economically viable addition to your new home.

In this country there are usually too many practical and financial disadvantages linked with constructing new basements which mean that they end up being very hard work and very expensive.

In the UK, when we need more room, we tend to either “build bigger” and take up more of the plot area for the building (or, in recent years it is becoming more fashionable to “build upwards”, – using the roof space to create extra bedrooms / bathrooms / family areas etc.

However, there are places, even in the UK, where a basement can make “sound sense”, both practically and economically. - These areas would include built up areas where plots sizes are a premium, sloping sites (where a bank is going to have to be dug away and / or fill brought in), plots with bad ground where either removing the ground and creating a basement, - or "piling" are the main choices.

I’ll start with “standard basements”:

Full basements in the UK are rarely built these days. They have never been particularly popular in normal housing in this country.

Why?

Well if you imagine an average building plot in the UK. Maybe 75 ft wide, by 100ft deep, flat, with houses either side. It’s big enough generally for a good sized 4 or 5 bed house with a garage.

Now imagine building a basement on it as part of a new home:

Firstly you would need to bring in excavation machinery. This needs to be able to move around the site, which in some cases will not be easy due to the limited free space.
Next, again because of the limited size of the plot, you would usually not be able to keep the excavated material on site (in the United States where plots can be far larger storing the excavated materials is often not a problem), so wagons would have to come and go from the site to remove all the excavated materials.
When you excavate materials from the ground they “Bulk”. - In other words they “loosen up” and basically take up about twice the volume that they did when they were compacted in the ground. - So, a basement for standard house could have to remove easily 20 to 30 wagon loads of material (or "spoil").
If you are paying £200 per day for the excavator and, say £180 a load to get the spoil taken away, then before you start any building work, you could find yourself "shelling out" around £7000 - just to dig a big hole! (how much of your loft conversion would that pay for?)
Add to that any problems you may hit, for example solid rock, the sides of the trench caving in, running water or sand, old foundations etc, - you may need to add another £2000 or £3000 onto that cost.
You will then need to support the sides of the trench before anyone can go and work in the excavation, - possibly another £1000
Then there is the cost of getting the basement designed and the calculations done for Building regulations.
Next you start to construct the basement itself, you start with a 50mm screed (thin concrete layer on top of the ground ), you need reinforcing, shuttering, damp proof membranes, concrete, probably a “concrete pump” to get the concrete “down the hole”. – THEN, once the slab is complete, you have to start on the retaining walls (which are a major task in themselves), followed by tanking, drainage, insulation and various other items.

The whole thing could easily cost you £30,000 and upwards.

And what does it give you? – A “big storeroom” underground where you may store your spare “stuff”, or use it as a games room. – Usually with little or no natural light, and possibly no significant “natural ventilation”.

“OK!” – You may say: “But we need the extra room for the growing family” – My answer? - “Go upwards instead!” – Design the house with rooms in the attic. - You will get the same extra floor area for possibly less than 20% of the cost, and have “natural light” and “ventilation” to boot!

If you don’t want to go upwards, why not try to use the building plot itself, to its “maximum potential?

See if you can enlarge the main structure of the house, whilst still keeping enough land for the driveway and garden. - Giving you the extra accommodation you need, within the 2 standard floors. – This would generally be the cheapest option of the three given here.

Where would a basement be a good idea?

Large, prestigious projects:

The only places in the UK where I would suggest that a standard basement could be financially and practically viable would either be:

In a “fairly grand” new property on a large plot, where budget would not be an issue and where maybe a wine cellar may be required by the owners, or a boiler room may be needed to run a substantial plumbing and heating system which may be part of the design.

These fact that the property was being built on a larger area of land, would mean that the excavated material may be kept on site, reducing the excavation costs.

If the overall design of the property is complicated and expensive, then the “unit costs” for the basement may start to come “within the range” of the general costs for the “overall development”. – If this is the case, then it may be something to consider ( but probably still not as a first choice!)

Sloping sites:

Especially in Scotland and Wales there are thousands of building plots which slope.

Some sites slope just a couple of feet, some a few metres.

“A couple of feet” of fall can usually be easily “dealt with” at the design stage, by “tweaking” the external design of the property, so that the ground levels can vary around the perimeter of the building. - This need not cost a significant amount of extra money to either design or construct. - Possibly just using a few hundred extra facing bricks and some extra blockwork where the ground is lowest.

It is possible to “design” quite a substantial slope out of a site:

Using maximum permitted gradients for driveways ( to comply with disabled regulations), you can set a driveway to fall at a slope of “1 in 9”, which can actually give you quite a lot of scope with the design.

If a property is going to be positioned 18m away from a boundary, then a 1 in 9 slope will allow the house to be set 1.8m higher, or lower than the boundary, without having to think about other options (like basements!)

However, when the slope of the site is such that it is approaching a “full storey” in height between one end and the other, then it may be time to consider using at least some of “the principles” of basement construction in the design.

For example:

If the site slopes “downhill” from the front boundary towards the back, you may come up with a 3 storey house design, which gives the appearance of a being 2 storey property when viewed from the front, and a three storey when viewed from the rear. – This actually entails building “half a basement” (see photo).

If the site slopes uphill from the front, it can result in a house which may possibly present a 2 storey property from the front, and a 1 storey property from the rear.

(Where I have seen this happen, the designers sometimes produce an “upside down” house, - where the living areas are on the higher floor and the bedrooms on the lower floor. This allows access to be gained from the living room onto the rear garden, and also often makes the most of “views” from the front of the upper storey.

The garage will be on the same level as the bedrooms, with the drive then sloping down from the house to the boundary.

The implications to you as a potential buyer, of “steeply sloping site” conditions, are that the build cost will go up significantly because of the need for the basement retaining structure.

The good news is that generally in the UK, the saving in the cost of the plot will often more than counteract extra cost of the build. - As a nation we are not “used to” building on slopes. - They tend to scare us! - So anyone selling land which slopes to any great extent, will usually put a realistic price tag on it.

So, if the site is sloping, and we want to build the extra “basement” storey: how does it work?

Quite simply:

The site is surveyed (see “Site Engineers / Surveyors” in the Trade Directory), so that all the “levels” around the site are known. From this information, the best position for the house within the plot can be decided.

If the site is Sloping downhill away from the front:

The house will possibly be designed to utilize a technique called “cut and fill”, with the “cut” being at the front of the property, and the “fill” being at the rear.

This means that the front part of the finished house will “sit into” the slope. - The bottom section of the front wall of the house will be a “reinforced retaining wall”, holding back the earth after that part of the bank has been cut away.

Then, as the ground level falls away towards the rear, the ground floor will of the structure will eventually show above the present ground level. The earth which is “Cut” from the bank at the front of the house can be used to “fill” the lower area towards the rear of the plot, to bring it up to the required level.

The effect of “cut and fill” is to give the appearance of a two storey house at the front, - under which there is a basement, but at the rear it appears as a full three storey house.

If the land slopes “uphill” away from the front:

Not surprisingly, the opposite happens.

The same system is generally used for the design. – The “cut and fill” method, but, this time the appearance of the “full extra storey” will be at the front, and the rear elevation will be one story less.

Sloping sites general:

Overall, if there is a choice between a flat site at a reasonable price, and a sloping site which appears very attractively priced, although the sloping site would probably be fine for developing, I would tend to stick with the flat site. - Sloping sites can bring extra problems which are not immediately obvious, such as the need to use piling for the development in order to pass the weight of the new building down to a “strata” which can cope with the load.

Also, one thing to remember if you DO develop a sloping site: Think about getting materials to the rear of the site. Often people plough ahead with the main building process, and don’t think about the landscaping or the drainage at the rear. Sometimes, by the nature of the slope, and the design of the new property, it is very difficult to get materials from the front to the rear of the plot. It may be worth taking materials you will need in a few months time for landscaping, right at the start of the job. – If they are covered and secure they should not come to any harm.

Conclusion:

So, - to sum up? – Basements in new Self Build projects?

If you are on a tight budget: Basements are probably something which will stretch your budget too much and this could have a detrimental effect on the project as a whole.

If you are designing and building something “prestigious” and the “unit” cost is not significantly higher than for the rest of the building, then they can be a useful addition.

Basement structures on sloping sites? – Can often offer the best solutions to the problem of how to deal with building on a slope. I have successfully used them recently on 8 properties on a steeply sloping site and they enhanced the property considerably, giving a full extra floor for a granny flat in addition to the two storey house above.

Many people use the opportunity a sloping site with a view gives them, to build "upside down" houses, with living accomodation upstairs, where the views are best, and bedrooms, utility rooms etc in the basement area.

If you “want one” – Have one! – There is nothing “intrinsically” wrong with basements, they are just not always practical or ideally suited to the UK. - Just do your homework on costs before you start.

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