Off-site manufacture (or OSM) is where the whole house – walls, floors, roof, windows, doors, electrics, plumbing, the lot – is manufactured in a factory and then effectively assembled on site. The method is exemplified by manufacturers like Baufritz, Hanse Haus, Huf Haus and, the newest entrant to the UK market, Kampa Haus — one of the biggest OSM companies in Europe, having built over 100,000 houses.

The raison d’etre of OSMs is to build more energy-efficient houses. As Baufritz puts it, “Efficiency: it’s a German thing.” And most of the OSMs are German, but we also have to add UK manufacturers into the mix: most notably Potton – famous for its Lighthouse but now launching the Zenit – and RuralZED, a company set up specifically to provide sustainable housing.

Of course, much self-build housing involves some off-site manufacture — most notably structural insulated panels (SIPs), where the panels are delivered to site with voids for windows pre-cut. OSM, however, delivers a finished house, up to and potentially including carpets and curtains. Diana Burns of Kampa Haus says, “We are stress-relievers. The client gives us a bare site and we give back a finished house. On time and on budget, every time. No stress, no pain.”

U-values

According to the Energy Saving Trust, a traditional brick and block house built to Building Regulations standards will consume up to 85kWh of energy per year, per square metre of floor area for space heating and hot water. The off-site manufactured house can get that figure down below 30kWh per year.

U-values are a measure of the rate at which heat escapes through a structure. So the lower the figure, the better. The Building Regulations call for a maximum U-value of 0.35 for walls, which would equate to brick and block construction with 90mm insulation in the cavity. Kampa gets that figure down to just 0.11 with single-skin timber frame construct ion with 300mm of insulation. Baufritz makes similar claims with 0.13 while Hanse offers around 0.15, and Build It Green 0.17.

Triple-glazed windows, heat pumps, heat-recovery systems, solar panels and pellet boilers are either included as standard or available options. In all cases the levels of airtightness are excellent, all achieving around 5m³/hr or better — twice as good as the current Building Regulations standards call for.

Taken together, these measures add up to huge energy savings. Hanse Haus suggests a typical annual energy consumption level of 43kWh/m², Baufritz 40kWh/m² and Kampa 24kWh/m², which takes into account a heat pump and heat-recovery ventilation system (compared to around 250kWh/m² in a typical house). Potton Zenit and RuralZED do not publish these figures but their insulation levels are very similar and, therefore, energy consumption should be about the same.

This translates in cash terms to the Building Regulations standard house costing around £50 per month to heat and the OSM house less than £20 per month.

Build Cost

Comparing like-for-like is difficult as OSMs quote prices to include the finishings, fixtures and fittings. The client chooses what to install and that directly impacts on cost. In addition, fluctuating exchange rates are affecting prices from German companies. As an example, Kampa was quoting £1,200/m² but is saying that this figure might need to rise to as much as £1,500/m² to account for the exchange rate. Potton suggests about £1,250/m² for its Zenit house and Baufritz quotes prices in Euros — €2,100 to €2,500 (£1,900 to £2,260 at current rates) for its bespoke turnkey service. RuralZED quotes £1,150/m² for a Code Level 4 home and £1,500/m² for Level 6, and Hanse Haus £700 to £900/m² for the shell only. There is also variation in what is included in the package. Kampa includes a heat pump and heat-recovery system as standard, but all the companies have their own list of standard features and optional extras.

By way of comparison, the Homebuilding & Renovating Build Cost Calculator shows average prices across the country varying from £900 to £1,600 for a medium-quality finish using a main contractor. While the figures do assume the delivery of a finished house, they do not, however, take into account budget over-runs, which are still a feature of the UK housebuilding industry. It seems that a traditional build is a little – but not significantly – cheaper than an OSM house. Maybe the price comes in bite-size chunks rather than all at once and so just seems smaller. The exchange rate is also having an impact.

Amanda Politzer of Baufritz says, “We offer a low-risk option and that includes the price. Right now we are encouraging clients to take the shell-only option and use lower-cost UK labour to complete the project.” This option, available from most of the OSMs, gives the benefits of energy efficiency and lower cost.

How Off-Site Works

The walls are built according to the computerised measurements; Windows and other openings are created in the factory

1. The walls are built in exact accordance with the computerised measurements.
2. Windows and other openings are created in the factory.

Each element is checked over in the factory before being packaged up for delivery to site; The house is delivered to site by lorry


3. Each element is checked over in the factory before being packaged up for delivery to site.
4. The house is delivered to site by lorry.

The front ground floor wall is craned on site, on top of the foundations; Each remaining wall is craned in


5. The front ground floor wall is craned on site, on top of the foundations.
6. Each remaining wall is craned in.

The team slot in smaller, craned-in sections; Once the first floor has been added, work on the roof can begin


7. The team slot in smaller, craned-in sections. The house fits together like a huge piece of flat-pack furniture, and is simply built upon in parts.
8. Once the first floor has been added, work on the roof can begin.

The finished kit house by Baufritz


9. The finished house by Baufritz.

Why Build an Off-site Manufacture House?

From an energy-efficiency point of view there is no contest: OSMs beat brick and block every time. If what you want is the hands-on involvement of building your own home then OSM is not for you. If you want an energyefficient house, delivered on time and on budget, then OSM may be the way to go. The reality is that by 2016, when zero-carbon houses are mandatory, we will all be doing something similar.

Working with OSM Companies

  • Work out the design process for your short – list of suppliers — some offer a fully bespoke service, while others offer plans off the shelf, limiting your design options
  • Don’t pay package companies large amounts upfront — insist the money goes into a separate trust account which protects it
  • Ask to see previous homes built by the company in question and get an independent recommendation
  • You might not need to sign up for the ‘whole’ package if you don’t wish to
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Comments
  • Nick Helder

    Please can you email me and tell me if you will be writing an update to this May 2009 article, its now April 2010?
    I am very interested in the various OSM companies and any advancement in efficiency values since May 2009.
    I have a fabulous plot of land 3/4 of an acre, with residential planning permission.
    A drawback of my plot is that it is in a Conservation Village and the plot was part of the garden of a Grade 11 Listed house that I sold in 2007.
    I have viewed examples Potton, Huf Haus and Weber Houses in 2005 but in my opinion the only design that I liked that may have passed West Dorset Planning would have been a Potton House and I wanted a contemporary steel frame, as near possible zero carbon house at the time.
    I would appreciate your help and guidance.
    Kind regards
    Nick Helder

  • Jason Orme

    Hi Nick

    It is a relatively fast moving area – companies are coming up with new wall systems and achieving greater efficiencies in terms of U-values all the time. Much of the advancement seems to concentrate on timber, however.

    Interested to know why you want a steel frame. Probably best if we know what the main priorities for you are – my view really is that U-values are so low now on most of the new systems that it’s in other areas – windows, airtightness, etc. that the key differentials lie.

    No immediate plans for an update of this, although Mark Brinkley and Tim Pullen do regularly deal with this subject in the magazine. Our June issue, for instance, features an interesting new solid wall timber system – one of the first houses to have it in the UK.

    Best wishes

  • Melanie Joseph

    Hello, I have permission to knock down my detached house in London and build a new home. I know I want to build a German pre-fab kit home and am pretty set on Baufritz because of their focus on creating a healthy home and protecting against electromagnetic smog. But, before signing on the dotted line I’d like to check out other pre-fab companies with similar health, Eco and efficiency standards, but I can’t find a site that compares pre-fab companies, is there one? And, if not could you provide a comparison of Baufritz and Hanse-haus (I know I DON’T want a Huf House as they don’t do electromagnetic smog protection and their focus is on Eco but not health). Thanks in advance for your help.

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