A sloping site is both a problem and a gift. The problem: it adds complexity and, therefore, cost to the project. The gift: when you’ve finished, you’ll have a much more interesting house. In this article we take you through the construction and cost implications for building on a slope.
How Will A Slope Affect Costs?
The use of reinforced suspended concrete floors – most commonly precast beam – and hard landscaping, extra drainage, including perhaps the necessity for a pump, and extra work in stepping the foundations will all cost money.
It is, however, questionable whether it is safe to try to extrapolate this into sliding scales or costs per square metre relating to degrees of slope — for instance 1% extra for every degree of slope. Average build costs per square metre are a rough and ready reckoner that you can use at the outset of your self-build project. They should never be used beyond the detailed drawings or once work has commenced on site.
Cut and Fill
This describes the process of carving out a level plinth on a sloping site, in order to build a home that is essentially designed for use on a level site. Any spoil that is cut from the bank is reserved in order for it to be brought back to make up the levels on the lower edge.
The foundation costs are always going to increase due to the slope of the land and the requirement that the foundations should find original subsoil bearing.
If the spoil is piled up against the lower or built up section of the new home then provision will have to be made for the oversite level within the building to be brought up to within 600mm of the proposed external soil level, in order to equalise the pressure on the walling.
However, carting spoil away from a site is expensive and time consuming and its retention on site is a cost benefit, just so long as there is space to store it.
Drainage and Sewers
Having a sloping site may involve extra costs with drainage and sewers — but not necessarily. If your site slopes down from a road in which the sewer is fairly shallow then you may have to think of using a pumped sewage system. This can add at least £2,000 to the drainage costs. However, there may be a corresponding saving due to the fact that the 50mm flexible pipe may well be cheaper to lay than a conventional drainage pipe.
If your site slopes down from the road, within which the sewer is quite deep, then the slope may actually represent a saving in cost, as the resulting house drainage will not have to be as deep. Sites which slope up from the road and sewer may seem more attractive so far as drainage is concerned, but if the slope is significant it might be necessary to install tumble bays within the manholes, in order to slow off the fall, so that the effluent can enter the sewer at a reasonable rate.
Surface and rainwater is also a consideration. Sloping up from the road may at first seem the best option, but many local authorities will not allow surface water to go into the public sewers and many require that precautions are taken to ensure that surface water does not flow onto the road. Sloping down from the road means that surface water can collect around the base of the lower floor, or worse still, find its way into the garage. This may mean having to install a drainage channel to divert the water to soakaways.
On a costing level a basement is always going to cost at least the same amount per square metre as any other part of the home, if not more. In ground with high water tables or in heavy clay, these costs and the sheer physical difficulties presented may make the choice unviable. However, if the lie of the land is such that there is no alternative to either a full or partial basement, then this can be the cost-effective solution.
Basement walls may have to be strong enough to hold back considerable banks of ground, in which case they become retaining walls in their own right. In other situations, such as building on a level plinth beside a natural or carved out bank, it may be necessary to construct separate retaining walls. Above 1,200mm in height these will have to be designed by an engineer.
In certain circumstances it may be cheaper, and visually more attractive, to construct a series of lower retaining walls with the ground stepped between them. An alternative is wire cages – known as Gabions – filled with stone, or interlocking concrete blocks that are subsequently filled with soil and planted.
Building On Stilts
One way of building on steeply sloping land is to build out from it on a series of supporting stilts or columns. This gets away from the need to build extensive foundations on sloping ground and it negates the need for tanking. It also leaves the ground relatively untouched, allowing planting to take place over much more of the site.
In certain situations it can be the cost effective solution and there is no reason why it cannot be employed with multiple level designs
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