In this quarter’s comparison of whole-life costs, David Weight of Currie & Brown digs deep into the financial pros and cons of adding a basement or half-basement to your building
Basements were once a common feature of houses built before the First World War, but their use declined steeply afterwards, partly because land was relatively cheap. They have never recovered their popularity, but interest is increasing, as available land reduces.
They have proved more popular in Europe. In Germany, development has been limited by footprint and height, so adding basements has often been the best way of increasing area without infringing planning.
For most buildings, adding a storey on top is likely to add more value than adding it underneath, but planning limitations frequently mean this is not possible.
The main concern with basements – less so with half-basements – is the complete or partial lack of day-lighting and view. Understandably, one of the main uses of basement levels is car parking, where the loss of daylight and view is unimportant.
Alternatively, a basement area may be used for hobby rooms, a gym, or more commonly for offices or storage. For document storage, depending on the type, importance, and required storage life, this may well require temperature and humidity control. An advantage of basements is the relative ease with which external noise can be excluded and internal noise contained. The famous Liverpool Cavern nightclub is one such example.
This article investigates full basements (fully submerged) and half-basements (partially submerged), and for the sake of simplicity, is based on taking a building of unchanging size, form and function, and seeing what happens as we sink it into the ground.
Live options: whole-life costing computer model
The whole-life costings presented here are based upon Currie & Brown’s Live Options software, which is an integrated suite of geometric, engineering and energy-calculation programs. Whether the development is an office, school, hospital or warehouse, the process starts in the same way, by using a basic but faculty-specific design template. This template is modified to incorporate, for example, changes in the size, shape, and operational performance.
The program integrates fabric and building services components and can show how costs change when a building element is changed, for example increasing the proportion of glazing in the facade. It does this by providing a detailed breakdown of changes, and shows the capital costs of physical items and the effect these have on building performance. Any changes to heating and cooling loads will be quantified in terms of the effect this has on, say, chillers and the electrical supply loads.
Changes to the energy profile of the facility are also calculated. The programme uses this data to assess the cost-effectiveness and overall viability of alternative solutions when considered against the facility’s projected economic life.
The model also takes account of repair and maintenance costs. This includes regular inspections, planned and unplanned maintenance and replacement costs for individual building components. These costs are automatically cash-flowed, discounted and summarized and net present costs are shown against each component, which assists in making choices and in value-engineering.
The floor over a basement must have at least 60 minutes fire resistance. Traditionally, there are three basic methods of constructing basement walls:
• Tanked protection, using applied waterproof finishes like mastic asphalt to a reinforced blockwork cavity wall with concrete infill behind.
• Waterproof construction using reinforced or prestressed concrete.
• Drained cavity construction using structural concrete to minimise water ingress, but with an internal drained cavity in the form of an inner skin to both wall and floor. This would lead to a sump and pumping arrangement.
We have assumed use of the first method, using 40 mm of asphalt applied in four coats.
The model assumes excavation and removal to tip for the main volume, and excavation to temporary spoil heaps and backfill for working space around the wall. Clearly, if excavation work turns out to be toxic or hazardous, then transport to the limited number of sites, and disposal costs would increase significantly. Alternatively, there are a large variety of chemical and biological techniques to treat soil on site. Examples of sites likely to be contaminated are: scrap yards, sewage farms, industrial areas, gasworks sites, and landfill sites.
Compared with building higher, building down has much greater risks. These include:
• Much greater influence of poor weather, with associated drainage difficulties and risks of flooding and delays.
• Poor ground and natural ground movement – seasonal or otherwise.
• Geological features, such as scour hollows.
• Limitations of noise, dust or vibration with associated limits on hours of work.
• Various types of obstructions, such as tunnels, pipes providing various services (live or redundant).
• Mining works.
• Antiquities such as archaeological remains. Such findings will normally require a postponement of the work.
• Graves are sometimes discovered in ancient cities in the UK. Where records are lacking, the likelihood may be indicated by proximity to old churches.
• Boundary issues, including nearby properties and their foundations.
• Loads from adjacent buildings and roads.
Generally, complexity increases with the excavation depth, a higher water table and a more congested site.
Normally, if space on site is available, basements up to two storeys deep can be constructed by just battering the sides of the excavation, sloping the sides of the excavation down to its base, meaning a support structure is not needed.
Good site information is crucial, including soil tests and a history of the site. The contractor should be wary of sparse ground-condition information, especially if taking on a lot of the risk. In most forms of contract, dealing with antiquities will entitle the contractor to both an extension of time and loss and expense, but the contractor should check this. Problems are easier to plan for and more cheaply dealt with, if identified before construction.
One way of reducing risk, is to use a new insurance-based scheme. The Concrete Centre, The Basement Information Centre and SpeCC – the registration scheme for specialist concrete contractors – have developed an accredited basement contractors’ scheme for members who are able to offer complete water-resistant basements with a 10-year insurance-backed guarantee. For example, a basement such as the one in the example below, costing £300,000, could cost 4% for the insurance, which is a one-off payment of £12,000 paid by the contractor, but covering the owner/occupier from day zero to year 10. The insurers are Guaranteed Pension Insurance Ltd.
02 Base building – naturally ventilated non-air conditioned business park office
- Gross internal floor area: 6,075 m²
- Form: 3 storeys high, and 15 m deep on plan
- Construction: Steel frame of 40 kg/m²
- Substructure: 200 mm ground-bearing slab, bearing on to 2 m-deep pad foundations.
- Roof: Flat, “Sarna” single-membrane proprietary roofing system.
- External walls: Mostly wall cladding with reconstituted stone. 10% is polyester powder-coated aluminium curtain-walling, fully glazed to clear height with double-glazed low-emissivity glass.
- Window frames: Polyester powder-coated aluminium double-glazed windows for about 60% of clear height using low-emissivity glass, with about 50% openable.
- Brise-soleil to windows facing near south.
- Conventional gas-fired heating, using a mix of radiators and trench heating.
- Lifts: One 10-person hydraulic passenger lift
Capital costs (£) (excluding fees)
- Substructure 307,871
- Structure 864,994
- External envelope 1,316,378
- Internal divisions 198,173
- Finishings and fittings 747,930
- Mechanical 760,989
- Electrical & communications 729,832
- Lifts 30,000
- External Works 523,525
- Preliminaries, overheads & profit 898,014
- Design & construction contingency @8% 510,216
- Total 6,887,922
Annual energy consumption (£)*
- Space heating 5,000
- Lighting 7,995
- Other energy costs 19,118
- Total 31,813
- and NPC (over 30-year period) £585,099**
- and an annual carbon load of 217,817 kg
NPC of maintenance and replacement over 30 years (£, limited to just the main elements which change most)
- Walls 13,675
- Windows 49,966
- Bris soliel 44,755
- Pumps for disposal installation 0
- Boiler 50,822
- Mechanical ventilation to basement level 0
- Lighting 153,952
- Total net present cost 313,170
* It is normal to use the same discount rate for all operating costs. However, energy costs are expected to rise more than general inflation, so if we were to use a 1.5% lower discount rate of 2.0%, the Net Present Cost for energy would be 21.8% higher at £712,477.
** discounted at 3.5%
03 Scenario one: forming a full basement – capital costs
Here, we take a three-storey building and push it into the ground to form a full basement, so losing one storey above ground.
To model the capital cost effects, just four inputs are required as follows:
• Change the starting level from zero (ground level) to one, keeping the same storey-height.
• Increase ground-bearing pressure for foundations because they are deeper. Bearing capacity generally increases with depth, partly because the weight of soil above helps prevent soil from squeezing out from underneath the pads and slab.
• Increase preliminaries to allow for a longer construction period. We have increased this by just 1.5%, assuming that about 80% of preliminaries costs are time-related.
• Reduction of air infiltration rate from 7 m³/m² at 50 pa pressure to 5 m³/m² at 50 pa. The maximum rate set in the 2002 Part L regulations is 10 m³/m², but it is not difficult to achieve 7 m³/m² in masonry construction. This initial value has been further reduced to 5 m³/m² because of the relative air-tightness of basement wall compared with the above ground envelope.
Changes to the capital costs
The capital costs changes in the graph (see below) are explained as follows:
• Substructure: purple represents the basement walls and excavation works for the basement, including disposal off-site. This includes the small insurance cost. Green indicates a small saving on pads and beams as a consequence of having higher soil-bearing capacity.
• Structure: the red indicates a small saving in the steel frame as a result of saving a wind-bracing element to one storey level.
• Envelope: blue indicates a saving on walls above ground, yellow indicates savings on windows, and pink represents the saving on bris soleil, which is assumed to be in the same proportion to the window area.
• Finishes: red indicates a small extra cost in the dry-lining system and wall finishes, because of the increased area of wall relative to the windows. This is because of the lack of windows at basement level compared with the level above which was lost.
• Mechanical: the main influence is the extra cost of providing mechanical ventilation to the basement level. Blue indicates a saving in the capacity and cost of the heating installation (see below). Red indicates the cost of pumps for sanitary waste disposal. Light blue show associated work to plant controls.
• Transport and builders work in connection with services: this is builder work in connection, which relates to M&E and has increased mainly because of the mechanical ventilation.
• External works: purple indicates water-proofing of service-duct entry chambers
Preliminaries have been increased (red), but the percentage for overheads and profit (light blue) is assumed to be unchanged.
The extra capital cost was calculated at £334,000 being £55/m² GFA.
04 Scenario one: Forming a full basement – repair and maintenance costs
The changes in the net present value of maintenance and replacement costs, discount at 3.5% p.a., is shown as follows:
- Walls -£4,538
- Windows -£16,655
- Bris soliel -£14,878
- Pumps for disposal installation +£5,581
- Boiler -£10,659
- Mechanical ventilation to basement level +£28,559
- Lighting +£24,267
- Total net present cost +£11,677
For the walls, we assume that there is no maintenance on the basement walls. It’s either that, or maintenance is very high. Maintenance to basement wall would certainly not be planned for and would be seen as a failure. Windows and bris soleil reflect the reduced area. The saving on the boiler reflects the reduction in capacity. The extra on lighting reflects in creased hours that they remain switched on.
05 Scenario one: forming a full basement - energy costs
Changes in energy costs are illustrated by the graph below. Prices for fuels are taken as 6p/kWh for electricity and 2p/kWh for gas, which is used for heating.
A further factor which had to be taken into account was a reduction in the average supply air-change rate. In starting, we assumed that the average air change rate in the winter would be about double what is required for comfort (typically 12 litres/person/second). This is because if users can open windows as they please, many will tend to open them to the extent that the heating system can cope without them feeling cold. This is undeterminable, but it is generally accepted that naturally ventilated buildings will have unnecessarily high air-change rates for much of the time. In the basement, however, we have a controlled mechanically ventilated environment, so we have reduced the assumed supply rate to 12 litres/person/second in this area, being an average of 20 litres/person/second throughout the building.
The changes are explained as follows:
• Fabric losses: blue indicates the reduction in area and heat losses from windows. Green indicates increased losses through walls, because the lost window area has been replaced by basement wall area. This has been mitigated by the fact that a basement wall will have a better thermal performance than an exposed wall of otherwise similar U-value (thermal transmittance). This is because, in approximate order of influence:
• Higher external temperature of soil compared with outside air. (The soil temperature at 3 m depth is set in the model’s climatic data and can be adjusted for different regions, altitudes, etc).
• Lack of influence of wind.
• Heat retention of deeper soil. The heat lost to deeper ground will tend to be retained, because it is further from the exposed ground, which of course, fluctuates more in temperature.
The above argument on thermal performance of the basement wall holds true for the ground slab, also (red). The effect increases with depth and with a reduced perimeter to slab ratio.
• Ventilation losses: blue indicates the reduced losses from unwanted air infiltration. The waterproof construction means that there will be no unwanted air infiltration through the envelope. Red indicates the saving in controlling air supply at basement level, compared with having uncontrolled natural ventilation at all levels.
• Winter gains: red is the extra costs from the loss of a gain, being solar gain from the windows, which have been lost. Yellow indicates a very small increased contribution that lights make towards the heating load
• Fans: green indicates the fans for ventilation to the basement level.
• General power: red indicates pumps, including for the sanitary disposal installation which would not have been required were it not for the basement.
• Lighting: red indicates general light, while green indicates task-lighting. It was assumed that the use of both are mitigated by good day-lighting especially where solar detection was assumed for lighting control. The loss of day-lighting to the basement level will clearly need more use of lights at that level.
The extra energy cost was calculated at £391 pa = net present cost of £7,191 at 3.5% discount rate, or £8,757 at 2% discount rate.
In practice, the use of sun pipes from roof level could provide some daylight to basements and this would mitigate use of lights. Sun pipes can include extract fans.
We have assumed identical functionality at each level, but in practice, if the basement is is used for storage space and meeting rooms, lighting may only apply to certain areas, intermittently. If this is so, the previously estimated extra cost on lighting will be too high, such that a predicted overall extra may even become a saving.
Whole-life cost changes
Whole-life costs over 30 years – the combined effects for losing one storey above ground and replacing it in a full basement, are:
- Capital cost changes + £334,000
- NPC of energy cost changes +£8,800
- Maintenance and replacement change +£11,677
- Total change in net present cost +£354,477
06 Scenario two: forming a half-basement – capital costs
Half-basements are a grey area, but for the sake of simplicity, we have dropped the ground slab by half of the depth taken in the first change scenario.
To model the capital cost effects from the base scenario, the following inputs were required:
• Change the starting level from zero (ground level) to minus one.
• The storey module at basement level is reduced and the same height is added to the ground-floor storey module. (This transfers wall from basement to above-ground wall, but adjusts the last for windows.
• Increase ground-bearing pressure for foundations since they are deeper. (between initial value and value used for first change scenario).
• Increase preliminaries to allow for a longer construction period. We have increased this by just 0.8% this time.
• The window/wall ratio is increased slightly, (from 60% to 65%) because the top half of the half-basement level will have a higher proportion of window than the above ground envelope.
• Reduction of air infiltration rate from 7 m³/m² at 50pa pressure to 6m³/m² at 50pa. because of the relative air-tightness of basement wall compared with the above ground envelope.
• While mechanical ventilation is added for the basement, the rate is cut down dramatically, as it is assumed that the openable high-level windows at basement level will be sufficient for user-comfort.
The items which have changed are all as change scenario one, but the extent has changed, being generally reduced:
The extra capital cost was calculated at £155,000, being £25.50/m² GFA.
07 Scenario two: energy costs
Generally, the trend is similar to the full basement scenario, but to a lesser extent. An exception is that no reduction has been made to the average supply air rate this time, since the half-basement is assumed to be naturally ventilated. Pumping for waste disposal has been left in, but there is a better chance that this wouldn’t be needed.
This time, a small saving was calculated at £73 pa = net present cost of -£1,344 at 3.5% discount rate, or -£1,635 at 2.0% discount rate. The main reason for the improvement from the full-basement option was the greatly reduced use of lighting and of mechanical ventilation.
08 Scenario two: repair costs
The net present cost of maintenance and replacement over 30 years (limited to just the main elements which change most)
- Walls -£2,912
- Windows -£4,826
- Bris soliel -£4,268
- Pumps for disposal installation +£4,000
- Boiler £6,222
- Mechanical ventilation to basement level +£5,712
- Lighting +£5,666
- Total net present cost +£2,850
Whole-life cost changes
Whole-life costs over 30 years, the effects for losing one storey level above ground and replacing it in a half-basement, are:
- Capital cost changes +£155,000
- Energy cost changes -£1,600
- Maintenance and replacement changes +£2,850
- Total change in net present cost +£156,250
The overall case for a developer goes beyond the scope of this article, because adding a basement is not a straightforward issue. Taken on purely economic terms, replacing a three-storey building with a two-storey one with the same gross floor area will increase the building footprint by 50%.
Adding external works, such as parking and access roads, including a basement, is likely to result in a 20%-30% reduction in land area required. If this is reflected in the costs of the land, the saving will usually be greater than the extra costs for basement construction. But most people would prefer to work or live at ground level or above, so this needs to be considered by the developer.
This desire for natural light can be partly addressed by putting the building in the ground and cutting away the sides so windows can extend down to the base of the building. This wouldn’t cost much more than a normal three-storey building, and usually with little or no loss in value, although this does depend on the specific site.