How Much Will Your Project Cost? - Part Two

Having an understanding of build costs is essential for any self-builder. It will help you to work out what you can afford to build, whether a contractor’s price is over-inflated or hopelessly optimistic, and the cost implications of each decision you make. A detailed budget plan will also help you to assess the overall financial feasibility of your project, comparing total costs with your home’s end value — something many lenders, especially the banks, will want to see before they will release any funds.

There are many factors that determine the cost of building your own home, from how much work you do yourself, to the level of professional involvement from an architect or project manager; from the shape, size and height of your design, to the quality of the fixtures and fittings you choose. Even the region you live in affects build costs because labour prices vary across the UK. This guide will equip you with an understanding of all of the factors, enabling you to plan your self-build project and weigh up your options to get the very best out of your available budget.

Read part one for advice on initial costs, project management and who to hire...

The Implications of Your Choices

You'll need to make significant adjustments to your budget to allow for specification choices.

External Walls

The external walls of your home represent on average 15% of the total construction cost. Most self-build homes are built using insulated cavity walls with an inner structural leaf of concrete blocks faced with brick, stone, hung tiles, timber or another concrete block and render. Building with standard facing bricks on 100mm concrete blockwork, with a facing brick costing £325/1,000 and 50mm partial-fill cavity insulation currently works out at around £110-115/m² of floor area.

Timber Frame: A significant proportion of self-builders use timber frame construction in place of concrete blockwork. The cost of a standard 89mm timber frame is comparable to the cost of blockwork plus insulation. If you decide to go for a highperformance timber frame, with thicker walls and more insulation (say 140mm frame), the cost will increase by around 10- 15% but with a considerably superior thermal performance.

SIPs: Structural insulated panels (SIPs) offer a high degree of air-tightness and insulation. The cost of standard 100mmthickness SIPs is more expensive than blockwork and insulation (manufacturers claim about 1%, although our experts suggest between 7-10%). Thicker panels offer superior energy performance but will add further to the cost.

Green Oak: Oak frame construction is especially popular in the border areas and the southern counties. It is a great way to create instant character in a new house but it is more expensive than concrete cavity walls or timber frame. Expect to pay an additional £40-60/m². You can reduce this cost by combining oak frame with SIPs, or masonry.

Insulated Concrete Formwork (ICF): This is reinforced concrete construction, cast on site using polystyrene moulds that remain in place as permanent insulation. This is a simple system and offers excellent airtightness and efficiency. ICF is marginally more expensive to use than traditional masonry, so allow an extra £20-60/m².

Cladding: The biggest variable in your external walls is not the construction system you choose, but the material you use for the external cladding. In the case of brickwork, prices can vary from £200/1,000 bricks up to £695/1,000 for handmades. It takes 60 bricks/m² of brickwork, so handmade bricks will add at least £28 to every face square metre of external walling, and probably more as handmade bricks are irregular in size and so take longer to lay, meaning labour costs are higher. Labour costs will increase further if you decide to have a traditional brick bond, such as Flemish bond, with courses of alternating headers and stretchers, rather than a straightforward stretcher bond. If you decide to add details such as stone window cills, heads and surrounds, or brick arches, these features will also increase your build cost considerably.

Adjusting for Cladding Types

*All costs per face metre (NB not floor area) except Details costs, which are per linear metre or fixed.

Upper Floors

The standard specification for first floors is to use softwood joists covered with flooringgrade chipboard. The cost is £18-19/m². If you opt to use engineered joists such as Ibeams, or beams with a steel web such as Posi-joist, this will add to your costs by £2- 3/m² but the more regular dimensions and improved stability can help avoid squeaky floors. Joists with an open web can help reduce the cost of first fix plumbing and heating, because there is no need to notch or drill joists — but this will only benefit you if you are employing the trades on a day rate, or doing the work yourself.

If you are building a masonry or concrete house you could opt for a concrete first floor structure, using either pre-stressed concrete beams infilled with concrete blocks, or large-format concrete floor panels. This offers superb strength and acoustic separation, and allows you to have masonry first floor walls throughout. This specification will add to your costs by £8- 12/m².

The Roof

Roofing costs include two main elements — the roof structure, which is typically built in timber; and the roof covering, such as slate, clay or concrete tiles.

The standard specification house includes £75.02/m² for roofing costs and is based on a shallow-pitched roof of just 22.5° spanning between two gable end walls, with trussed rafters made in a factory and large-format concrete interlocking tiles (£1,700/1,000 at 8.2 tiles/m²), plastic gutter and downpipes, and 200mm of quilt insulation. This is about as cheap as it gets, because the lower roof pitch uses minimum timber (shorter rafters) and the roof has a smaller surface area, using less labour and materials for insulation and roof covering. Engineered roof trusses, held together using gangnail plates, are also very cost-effective.

The good quality roof has a steeper roof pitch at 30° which will look more appealing, but makes the rafter lengths longer and so adds to the cost of the trusses — and creates a larger roof area. It also uses more expensive machine-made clay interlocking tiles (£900/1,000 at 20.5 tiles/m²) adding to labour and material costs for roof tiles, felt and batten and insulation, resulting in a cost of £93.77/m².

The excellent quality specification has the same trussed roof but a steeper pitch at 35° and more expensive plain clay roof tiles (£350/1,000 at 60 tiles/m²) pushing the cost up again to £112.52/m².

Roof Pitch The steeper the roof pitch, the larger the roof area and consequently the greater the cost. As a general rule, every 5° increase in roof pitch adds 4% to the roofing cost £/m².

Roof Structure: Changing from a standard ‘fink’ trussed roof to engineered attic trusses will increase the cost by £1,500- 2,500 for the additional timber. A traditional ‘cut’ roof (built on site) would cost roughly the same as attic trusses. Using SIPs for the roof structure in place of trusses is fast and creates maximum useable roof space with a high level of airtightness and thermal insulation. Costs are likely to be slightly higher than for a traditional cut roof or attic trusses.

The cost guides given in the tables are for simple duo-pitched roofs with few junctions. Adding hips, valleys and dormer windows will increase the average cost.

Adjusting for roofing finish

*costs quoted per facing m² (NB not floor area)

Roof Covering: Your choice of roof covering will have a major effect on your construction costs. Speculative builders favour large-format concrete interlocking tiles with a coverage as low as 8-10 tiles/m². Most self-builders choose to use more attractive tiles made from natural materials such as clay, slate or stone, which require more tiles per square metre, thus increasing costs. If you plan to use a roofing material other than that allowed for in the average costs (£40/m² standard, £50/m² good, £80/m² excellent) you need to add an allowance for the net difference in cost. Adding decorative details to the ridges or verges will add to the overall cost.

Foundations and Ground Floor

The cost of foundations is one of the few aspects of housebuilding that you cannot accurately predict until you actually start digging. Unlike other aspects of your build, where you are in total control of the specification, it is the Building Control surveyor who will ultimately decide which foundation solution to accept for your site.

Unlike most costs, which vary according to the quality and size of house you are building, foundation costs per square metre are likely to be more or less the same whatever you build.

Most sites with good ground conditions would be suitable for traditional concrete strip foundations (1m-deep trench, 600mm width, with at least 225mm of concrete), with a reinforced concrete ground slab, insulation, polythene dampproof membrane (DPM), plus blockwork below external ground level and blockwork with facing brick cavity walling up to damp-proof course (DPC). The average cost for foundations and ground floor is £79-81/m², some 7.5% of the total construction cost.

The cost of using trench fill foundations (less below-ground blockwork but more concrete) or a suspended concrete (beam and block) ground floor structure are much the same, both requiring the same insulation and either a minimum 65mm screed finish or a 22mm flooring-grade chipboard floating floor. Variables will be the proximity of your site to the nearest ready-mixed concrete plant, and the nearest tip for removing spoil.

If your site demands a special engineered foundation solution, such as a raft, piles and reinforced ringbeam, or a reinforced slab, allow an extra £10,000 in your budget.

Originally published November 2008