How to simplify your build
Overengineering is a common issue on major home projects – and one that can be difficult for self builders and renovators to identify. Tim Doherty reveals some of the key areas you can look to maximise value
Tim Doherty reveals his top tips to make sure your project doesn’t spiral out of control and keeps within budget
What do we mean by overengineering? Well, in terms of construction projects it refers to buildings where some of the components being specified (or assumed) are adding very little extra value to the finished house. In some cases, a project might have become so complex that a simplification process is necessary to reduce the build cost and bring the budget back under control.
You can stave off this possibility by ensuring your design goes through a phase of value engineering. This is where you and your build team interrogate the structural plans to question why certain components have been included, identify what would happen if they are removed and determine whether there is a more straightforward way of delivering the home you want with some small tweaks that could make big savings.
It’s important not to lose sight of your goals, however, so keep in mind the need to retain the wow factor and character that you want. A good architect may well have included specific details that produce well-balanced and proportioned structures, which look great because of certain subtle features. The trick in your value engineering phase (and during the phase itself) is to know what these elements are and to select and synthesize your materials, so that the important design elements are protected.
You’re the client, so the final decisions will be yours. When carrying out your value engineering, then, you’ll need to balance affordability with style and, ultimately, the building’s finished appeal and end value.
Who should lead the technical design?
Your architectural designer will set the tone for your project. At the planning phase, their drawings may be quite sketch-like, and some will hand-draw or colour their floorplans and elevations as part of a successful submission. CAD (computer aided design) drawings can be more clinical, which may not always be ideal for a planning application, but they do offer a 3D design option that can really help in understanding the structure.
This is why developers working on larger commercial projects will almost always use sophisticated design
software. This enables them to optimise the building and balance affordability with delivery.
To prevent shocks further down the line, it’s worthwhile having your architect walk you through the design. This will allow you to understand any subtleties, such as where and why there may be changes in floor levels, where the architect intends beams to be included, how the roof is configured etc. On complex dwellings, your architect may have already had preliminary discussions with a structural engineer – or, at the least, they should have an engineering solution in mind to satisfy the layout.
For true one-off houses, however, it’s the detailed (aka technical) design and Building Regulations application stage that will reveal the full extent of what’s going to be needed. The most recent individual project I was involved with looked, on the face of it, to be quite a straightforward house of around 300m2. The architect had secured planning, and it wasn’t until the Building Regs phase that the full extent of the steelwork requirements became apparent. The project required no less than 26 steel beams, five of which were cranked (where the beam includes two kinks to turn through an angle), and six steel columns. There was no indication from the designer that this level of steel would be necessary, so the cost of engineering came as a bit of a shock to the client.
The structural engineer is the ultimate arbiter of any structural queries, as it is their calculations that will justify the design. Building control will review the proposals and set out to ensure compliance with the regulations and, whilst they may query structural issues, they will more often than not defer to the engineer’s calculations. So, the architect, engineer and building control officer are the three primary bodies who will assist in a design-led valueengineering process, all of whom should be engaged long before you start on site. If you’re trying to value-engineer with work already underway, then it’s likely something has gone wrong somewhere.
Contractors can also be very helpful in bringing a practical emphasis to the discussion, in querying how a certain detail is to be constructed or by offering an alternative approach. In fact, the value engineering process frequently starts as a result of tendering a project (calling in quotes) from target contractors. This is a common time when clients find out their design is too expensive and that costs need to be trimmed.
Value engineering foundations
It’s always worth scrutinising the foundation design.
The default position is frequently to opt for 600mmwide trenches and mass fill these with concrete down to the appropriate depth. But if you have good load-bearing strata on your site (say 100kn/m2) and assuming your house line loads are fairly standard, then a foundation width of 450mm might be sufficient. It’s slightly trickier to set out on site, but perfectly adequate.
With poor soils and where standard foundations might have to be 2m or 3m deep in some areas, it often makes sense to move away from trenches. In such cases, an engineered raft foundation solution may be more cost effective. Alternatively, a combination of small piles with reinforced ground beams can help get over difficult terrain and – while they might look expensive on the face of it – could represent good value for money. So, it’s always worth exploring sub-structure options in detail well ahead of actually starting the works on site. Check out www.self-build.co.uk/soil-survey for more.
Simplifying the superstructure
Some building systems lend themselves to certain types of design. For example, if you have wide spans of standard height external walls, big areas of level intermediate floors and long-but-simplistic roof pitches, then SIPS (structural insulated panels) and cassette floors may prove to be an economic solution. Especially when you consider all the wider implications, such as greater prefabrication, less work on site, simple cladding options etc. This very system could, however, prove to be an expensive option for more complex geometric shapes where nothing is standard and no two elements are the same. So you might have your heart set on a solution like SIPS, only to find it’s not the most cost-effective route for your design.
The trick is to keep an open mind all the way up to the engineering stage and then seek good advice on the ideal solution. That should include talking to suppliers, as well as those who don’t have a vested interest in a particular system. Blockwork, timber frame, SIPS and ICF (insulating concrete formwork) will all deliver pretty good energy efficiency and can comfortably exceed the Building Regulations requirements. So your ultimate selection should be based upon cost, site logistics and your own nuanced preferences in terms of solidity, acoustics, sustainability, airtightness, resale appeal etc.
External wall finishes
Some cladding options are simply decorative rainscreen layers, rather than making a contribution to the structure
or thermal performance of the building. With SIPS, timber frame, ICF and solid (single-leaf) concrete blockwork construction, finishes such as timber boards, render and vertical tiling can all be directly applied – so there’s no need for a discrete masonry skin (as with cavity walling). This reduces the overall weight of the building, lightening the load for foundations and eliminating a skin of blockwork in the process. When specifying lightweight claddings, window reveals and internal and external sill details need to be thought through. But a big cost saving can be made with not too many visitors noticing that your house is made from thinner walls.
Where this approach can sometimes go wrong, however, is in getting the details right when mixing and matching. For instance, if you use lightweight claddings for the first floor but don’t compensate for their thinner profile by pushing the structure out slightly. This can have the effect of putting a belt around the building’s waist and creates an awkward and unsightly flashing detail. This is something you might not appreciate until you see the finished article.
Don’t rule out a basement
A basement can be a fabulous addition to a house, and many self builders have regretted not creating one when they had the opportunity. The Americans and Canadians love basements, and it’s common practice to simply repeat the whole building footprint under the ground floor. We’re a bit more hesitant in the UK, and don’t tend to like the approach having a few steps up to the front door.
Nevertheless, subject to a positive soils report, it does make great economic sense to include a full or partial basement – but only where the accommodation is fully integrated and useful. Think things like cinema rooms, gyms, home offices and guest suits. In other words, habitable rooms that require features such as windows (possibly created through lightwells) and a proper means of escape. Regardless of the size of your proposed basement, try to ensure it’s positioned to match up with load bearing walls so you don’t need to duplicate any superstructure supports. If the space is useful and doesn’t just become a dumping ground, then ultimately it should deliver value for money and bring the overall cost per m2 of your house down.
Kitchen & bathroom layouts
If possible, try to work your floorplan so bathrooms,
WCS, kitchens and utility rooms are all on one side of the building. This will greatly simplify the design and installation of your above and below-ground drainage. It may not always be doable, but thinking this through at the initial design stages could take away a lot of headaches. Adding extra soil stacks, drainage runs and manholes at every change of direction can quickly hike costs – and I mean by thousands rather than hundreds of pounds – so it’s well worth considering in the early stages.