Part Twenty Nine – Some Details That May Be Of Interest

This twenty ninth entry was published originally by JSHarris on the 23rd April 2014 and received 2,303 views on the closed forum

I’ve been asked a few questions about technical details and costings, so thought it might be useful if I added a bit of detail.

First of all, how much has it all cost?

Our site costs were high, and not very typical of most sites, so what I’ve done is break down the other costs and add in the basic ground works cost, just for the house alone (excluding the garage), to give an idea of how the costs break down. Bear in mind when looking at this that there is a fair bit of DIY in there; I was the architect, project manager, first and second fix plumber, ventilation and heating system installer and have done a fair bit of other stuff.

Simplified costing spreadsheet – 050421014

So far the total looks like it’s going to come out at around £173k just for the house and basic landscaping. This seems reasonable for a house that comfortably exceeds passive house performance levels and is reasonable well finished internally (we could have saved a fair bit by not opting for oak doors, staircase and joinery, and going for a cheaper spec for the kitchen and bathrooms). Hopefully this might be of use to others.

Next I thought I’d add some details of the technical stuff. Dave (ProDave) asked how I’d fitted everything into one meter box, so here’s a photo:

It’s a tight squeeze, but everything fitted in there (just). The lower box is a modified caravan hook up box, that I used as a temporary site supply initially, but is now permanent. It has four RCBOs, one feeds the blue 16A socket, one feeds the treatment plant the third feeds the borehole water pump and the fourth feeds the garage, which is TT’d locally, rather have the earth exported out over the SWA. The two way CU in the lower right corner just  feeds the waterproof CU below, as an isolator. The 80A fused isolator in the top right corner feeds the 25mm² SWA that runs in a duct under the slab and up to the services area in the “loft”. The garage has its own RCD protected two way CU, one circuit for lights, the other a ring main running a lot of metal clad sockets all around the walls, but the earth for this is TT’d, just for my peace of mind.  The 6mm two core SWA that feeds it is fed by a 40A DP RCBO, so the underground cable is well protected, too.

The 25mm² SWA from the meter box terminates in an adaptable box in the upstairs services area. The tails then run out to a meter that I installed simply because it has a data port that will allow me to monitor and log the energy consumption of the house. The PV import meter is right next to my monitoring meter, which is next to the internal AC isolator for the PV system. The consumer unit has RCBOs on every circuit, something very well worth doing for several reasons. Firstly, it means that a fault will only trip one circuit in the house, which is less inconvenient and makes it a bit easier to find the fault. The second very good reason is safety related. If you have a PV installation then the inverter can continue to supply power to the consumer unit for up to 5 seconds after the mains supply is disconnected (say by an RCD in a conventional 17th edition split board). This can then, potentially, create a situation where the RCD trips because someone’s had an electric shock (and they are designed to trip fast to prevent death, 30mS) yet the PV inverter carries on supplying power to the side of the board that’s tripped out for a few seconds. My personal view is that this is very iffy, but an all RCBO board circumvents this risk, even if it is a fairly small one.  There is a DP time delay RCD as the master, just to catch the near-impossibly rare faults that a SP RCBO won’t always catch, but as my electrician said, it’s total over-kill for the sort of installation we have.

This black box:

is the home made PV diverter. This works like an Immersun Mk 2 and diverts excess power from the PV array to the thermal store (the green thing to the right). Whenever the PV power generated exceeds the house power demand the excess is fed to the immersion heater in the thermal store, providing most of the hot water for the house, free of charge. We still get paid for generating power, on the basis of 50% of the deemed generating capacity of the PV system, whether we export that power or not, so it makes financial sense to use it, rather than export it. In practice we’re still going to export a fair bit of power, as even on a dull and rainy day like today we generated around 8 kWh, more than we’d need for hot water.

The UFH heating isn’t filled and commissioned yet, but the pipework and controls are all finished and tested. This is the weather compensated, sensitive thermostat I made to control the circulating pump:

The thermostat measures the outside air temperature, via a probe on the North side of the house, and the slab temperature, via a probe set near the centre of the slab. It has a switching threshold of +/- 0.1 deg C and is designed to hold the slab at an accurate temperature of around a deg or so above desired room temperature. It does weather compensation by increasing the slab temperature by 0.1 deg C for every 1 deg C drop in outside temperature below 5 deg C. I can change the slope of this characteristic and anticipate I may need to fine tune it to get the house temperature to remain stable. I had to go down this route because a conventional thermostat would have had far too great a hysteresis (the difference between the turn on temp and turn off temp), and could have resulted in large swings in room temperature (one of the problems with controlling a passive house is supplying just then tiny amount of heat needed, very accurately).

Finally, here’s a bit of the kitchen bling:

This is an Itho stainless steel boiling water tap. The right hand lever is a normal hot/cold mixer, fed from the hot and cold supplies, the left hand knob (with a safety button) is the boiling water dispenser. This means that we have no need for a kettle, and can make tea straight from the tap. The boiling water is provided by a very well insulated pressurised boiler that sits under the plinth:

It’s fed with softened water by a small under-sink cartridge water softener, on the left in this photo:

Getting everything neat and tidy under the sink was a struggle, as I wanted to try and leave as much useful cupboard space under there as I could. Being able to put the boiler unit under the plinth was a great help, as it would have taken up loads of space in the cupboard. I may swap out the switch for the boiler for a time switch, so that it’s only on when we need it, which is mainly during the day (when the PV will, hopefully) be generating. It only uses 32 Wh of energy on average, though, so wouldn’t be a massive cost even if we left it on all the time (especially as we’ll still be net exporters of power over the year,anyway).

I should have some photos of the completed water treatment plant soon, it’s nearly finished, and with luck will be commissioned tomorrow.


ProDave 23 Apr 2014 07:13 PM:

 Excellent stuff.

 That costing spreadsheet is a great help for costing my build. Since I’m going down the route of getting one person to design it and then another to build it, with me doing all the finishing off, it will be a bit hard to cost. And the first time I’m going to need an accurate costing is for the building warrant application, where the fee is based on the cost of the building work.

 And yes there’s a lot in your single meter box. Did the meter monkey say anything? sometimes they get a bit uppity if they think there’s too much stuff in “their” meter box.


 slidersx200 23 Apr 2014 07:24 PM:

Interesting reading as usual Jeremy and once again you have been very open in sharing your experiences of the actual costs associated with specific elements of the build. How did you fair with the PV system if you don’t mind me asking? It’s currently on our “if we have ANY money left” list, but I’d like to ensure the heating/dhw system is designed to take advantage of it in the event we do fit it someday.

 Slightly off topic perhaps, but your mention of the temperature probe in the slab got me thinking about how much of the plumbing, electrics etc need to be installed before the slab is poured. I know quite a lot can be accommodated in the service void or internal stud walls, but no doubt some items will need to go through the floor and as (if I understand correctly) when using the Viking House/Supergrund type systems the finished floor surface is poured before the frame is erected, quite a lot of decisions will need to be made very early in the construction stages.


 jsharris 23 Apr 2014 08:03 PM:

 Dave, the meter guy had a moan, but I just said that it was my box, I’d paid for it and installed it, and that there was loads of room for the company head and meter (this was before I’d fitted the fused isolator at the top right…………….).

 Suzanne, you really need to think ahead and get as much as you possibly can come up through the slab. This means a lot of very accurate setting out, as you need to be certain that the soil pipe(s), water pipe, electricity supply, phone cable etc are all in the right place inside the house. I opted to have just a single soil pipe stack come up through the slab, by deliberately designing the house so that all the toilets (on both floors) were located near to a single point, which meant having to only have one waste pipe (for the kitchen sink and dishwasher) go out through the wall. This significantly reduced the heat loss from having too many holes in the building.

 The temperature probe was added afterwards. The MBC guys marked where the UFH pipes were (plus I had photos) so it was easy to drill a hole into the slab in a safe place and grout in the probe. I did this as part of first fix electrics, and fitted two sensors, with the cable from the spare one coiled up in the eaves (in case one of the sensors fails).

 PV is an absolute no-brainer. Just do it. Ours is generating loads of power (it’s now over 700 kWh since going live on 20th March). It was also relatively cheap, certainly given the amount of energy it will produce over its lifetime, plus the FIT payment. The pay back time, just on the FIT payment, will be around 7 years, and that ignores all the free energy we’ve made use of for hot water.


 joiner 24 Apr 2014 06:59 AM :

 All good stuff!


 coopers 24 Apr 2014 09:57 AM :

 “Suzanne, you really need to think ahead and get as much as you possibly can come up through the slab. This means a lot of very accurate setting out, as you need to be certain that the soil pipe(s), water pipe, electricity supply, phone cable etc are all in the right place inside the house. I opted to have just a single soil pipe stack come up through the slab, by deliberately designing the house so that all the toilets (on both floors) were located near to a single point, which meant having to only have one waste pipe (for the kitchen sink and dishwasher) go out through the wall. This significantly reduced the heat loss from having too many holes in the building.”

 I think this was meant for Sliders (Michael), who’s not wearing his dress today.

 All good info J, and really useful for all us would-be-passive-house dwellers. I’m sure you’ve done the energy efficiency calcs on boiler tap vs kettle?


 jsharris 24 Apr 2014 05:18 PM:

 Sorry Suzanne and Michael – I blame it on one too many glasses of “relaxation” after a particularly busy day……………


 wmacleod 24 Apr 2014 08:30 PM:

 “ProDave, on 23 April 2014 – 07:13 PM:, said:

 And the first time I’m going to need an accurate costing is for the building warrant application, where the fee is based on the cost of the building work.”

 Dave, I don’t know if you will be allowed to use your own costings for the work, I certainly wasn’t when I questioned it, I had to pay the fee based on £1136m2 for the ground floor and £813m2 for the upper floor unless a certified chartered surveyor would go over my costings and sign off on them. Fees for that would have been more than the difference in the building warrant fee!

 Pictures look great Jeremy, it is really interesting to see the build progressing so well. The breakdown of the costings is very helpful as well.



Alphonsox 25 Apr 2014 09:15 AM :

 Many thanks for sharing so much detail. It is proving extremely useful for getting our build up and running.

 A couple of questions…

Does the £53K MBC cost include the cost of the garage timber fame ?

What controller did you end up using for the UFH ? some form of PID ?


 jsharris 25 Apr 2014 04:16 PM:

 No, that was just the cost of the house foundations and frame. The garage frame and insulation was about £7k, and the slab and ground works for it was another £1600. The garage is 6.1m x 4.1m and is lined and insulated, with a floored out storage loft, so I can use it as a workshop.

 The UFH controller is just a simple on/off thermostat, with a +/-0.1 deg C switching threshold. The time constant of the slab is such that there didn’t seem any merit in doing anything more sophisticated, although it’s very much an experiment, so I may be wrong. All the controller does is turn the UFH pump on and off. The UFH flow temperature is fixed via the mixer valve (which can be adjusted to give me control over the warm-up time). The calcs show that with a 25 deg C flow temperature the warm up time should be about 1/10th of the cool down time at maximum heat delivery, so I’m hoping that this simple approach will do the job.


 coopers 28 Apr 2014 07:01 PM:

 Hi Jeremy,

Was the house fully battened ready for tiles? Today I spoke to our preferred builder, who will be doing all the non-timber frame details. He expressed concern that the roof will be felted and battened. He said it’s a recipe for disaster, as if the battens aren’t properly aligned, the roofers will have to rip it all off. He said if they are fly battened, then it’s ok. I’ve tried to look this up, but I’m not getting anywhere.


 jsharris 28 Apr 2014 07:23 PM:

 Yes, it was fully battened exactly to the right spec for the slates we used. I just ensured that MBC had the correct batten spacing, including the shorter spacing for the first batten (to allow for the 50mm overlap in to the gutter).

 Not at all a problem, the roofers had no problems at all with fitting the slates properly, as all the battens were exactly where they should have been.

 I’d question the wisdom of your builder, as he clearly doesn’t seem to understand quite how accurately built one of these houses is (and needs to be). As another example of accuracy, I double checked the finished dimensions of the downstairs WC last week (in order to order units). I designed it to be 1210mm wide (to allow for two 600mm wide units with enough room to fit them and allow for any slight errors in squareness). The room turned out to be 1211mm wide with dead square corners.

 Do NOT accept it being fly battened, the battens need to be fixed on the correct centres to the counter battens at the first go, as they are nailed through the special breather membrane (it’s not felt). All you need to do is let MBC know the pitch required for your chosen roof covering (including the offset lower batten) and they will do the rest, to a very high spec.


 coopers 28 Apr 2014 08:21 PM:

 ah, I see, that makes sense. He was saying that if a house isn’t square, it will show in the tiles more than anywhere else. I guess he’s used to building wonky houses!!

 Thanks Jeremy



28 Apr 2014 08:39 PM:

My experience has been that pretty much every trade I’ve had on site has been amazed at how accurate and square the house is. British builders are clearly more used to houses being built to much poorer levels of accuracy, which undoubtedly leads to many of the problems we have with poor airtightness and thermal detailing with so many British built houses.


 eddleetham 29 Apr 2014 06:58 PM:

 Jeremy, I’ve been struggling with making comparisons between different timber frame manufacturers’ quotations, as they all seem to have slightly different specs. When I spoke to Seamus he helpfully suggested that his quote would be roughly half the cost of the finished building. It’s obviously only a ball-park figure, but would you agree that for a straightforward build without any major site problems, and assuming an average quality of finish, that’s a reasonable estimate?




 jsharris 29 Apr 2014 07:29 PM:

 Hi Edward,

 I think that, for a very basic finish, then it might “just” be possible (at least in cheaper labour cost areas of the UK) to build for this sort of cost, but it would mean a fair bit of DIY.

 I think it’s pretty hard to get any self build in the UK, especially in the south, to come in under £1000 sq m, and most average builds seem to come in between £1100 and £1300 per sq m. The cost of our basic foundation and insulated frame from MBC Timberframe came in at around £400 per sq m; others here have had quotes for simpler builds from them that have been closer to £350 per sq m.

 I think there’s no substitute for doing a detailed costing, including as much detail as you can. One major problem I found when trying to do this, was that many of the frame suppliers were very unclear as to what was included or not included. More than once I thought I’d found a bargain, only to find that when I costed in all stuff that wasn’t included the true price was way over the attractive quote.

 My feeling is that the finished cost is going to be around 3 times the foundation and insulated frame cost, even for a fairly basic finish.


 coopers 29 Apr 2014 07:39 PM:

I agree with Jeremy, at least 3 times the frame/slab/insulation costs.


 TerryE 10 May 2014 01:11 PM:


 Since I will be doing my own spreadsheet, I pasted yours into OpenOffice Calc and rechecked the totals to make sure that I hadn’t made a mistake. In doing so, I noticed that you’d missed out a subtotal for Floor coverings, which carries through to your grand total. Shock, horror, I’ve just added £4,976.41 to your total build cost. Sorry 

 But v. useful. Thanks.



10 May 2014 01:40 PM:

Well spotted!

 Thankfully it’s only on the cut down and heavily edited version I posted here, the main spreadsheet I’ve been using has that sub-total included. Somehow I must have accidentally edited out that sub-total when producing the version I uploaded here.

 I’ve now edited the version in the post above to reflect the true cost!



 pocohontas 16 Sep 2015 01:28 PM:

 Sorry if you’ve written this elsewhere – did you get the Vat back on the MVHR? I’m looking at getting a few things from Europe.



 jsharris 16 Sep 2015 06:45 PM:

 I’ve not put the claim in yet, but I did call HMRC and they confirmed that I could claim the Danish VAT back, as long as I included evidence of the exchange rate at the date of purchase.

 Others here have claimed the VAT back on purchases made in the EU. I think Colin (temp on here) claimed back the VAT on stuff he bought in the EU.


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