How much does a basement cost? The budget costs below include soil investigation, structural design, excavation, working room, blinding, forming the structure, concrete, reinforcement, waterproofing, labour, plant, wall insulation and backfilling. In other words, all the costs extra to building a house without a basement. Most basements are built for around £300 a m² of floor slab plus £300 a m² of wall area less the savings for foundations. A simple structure built into a slope where less soil needs to be excavated might come in 15% less. Expensive temporary works close to neighbouring structures, say within 1.2m, and particularly wet ground or the removal of contaminated soil can increase this budget figure. We find most designs come in over-engineered and they would cost more but we send them back with our suggestions regularly saving clients tens of thousands of pounds. Likely overall cost of a basement with internal dimensions of : 2m x 2m - £12,125. £3,030 per m2. 3m x 3m - £18,100. £2,010 per m2. 6m x 3m - £26,800. £1,490 per m2. 7m x 5m - £36,700. £1,050 per m2. 10m x 7m - £58,900. £840 per m2. 12m x 10m - £86,300. £720 per m2. 16m x 12m - £123,300. £640 per m2. 20m x 20m - £219,000. £550 per m2. 40m x 10m - £240,000. £600 per m2. These figures are for the whole waterproof basement structure including professional investigation, design and excavation but not for any internal walls, first or second fixing or the floor above. The savings on foundations should be deducted from these figures to give a net cost. Basements benefit from economies of scale, so the bigger the basement the more cost-effective it is for each square metre of accommodation gained. Economies of scale: Basement walls replace house foundations. So the more the basement footprint matches the house footprint the more you replace and save money. Ideally, therefore, the basement will be the same size and shape as the house above. Basements all require the services of a structural engineer and a professional soil and water level investigation. These are relatively fixed regardless of size. Basements consist of excavation, drainage, floor slab and walls. As a basement gets larger, so its ratio of walls to floor area reduces, meaning proportionately less drainage, less wall and less excavation for the area gained. A very small basement will need small, inefficient loads of concrete and small, inefficient use of concrete pumps. We have added a realistic surcharge to the £300 a m² to the smallest 3 sizes above to fairly reflect these inefficiencies. In our experience, having a basement only half the size of the house above only saves a quarter of the cost of having a basement the same size and footprint of the house above. (So to double the size of a basement currently planned to be half the size of a house would cost only a third more but might save 3 times as much on foundations sometimes cancelling out all the extra cost of having the basement twice as large). Please contact us for an estimate of the cost of your project and our comments on designing in efficiencies. We cannot supply a firm quotation without a professional soil and water level investigation since soil and water conditions make a considerable difference. Open our sitemap in a new window should you need it. For instance, to make printing pages easier or to see which pages you have already visited.