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SIPs/ prefab extensions - anyone done one?

(13 Posts)
narmada Sun 26-Jun-11 14:23:53

Hello, looking for some advice from any experienced home renovators out there.We are hopefully soon to exchange on a solidly-built (we hope) 50s' built ex-council semi. We'd like to extend to create a bigger kitchen/ diner/ family room and a new bedroom over the top. There's plenty of room around the property (large garden and side return) and neighbours have done similar.

I was wondering about the feasibility of doing the extension in an unconventional way - in particular, using SIPs (structural panels) or some other prefabricated building materials.

let's just imagine the planners gave the green light hmm. How durable are these extensions? Would we have problems when we came to sell due to having put an unconventionally-built extension on?

I'm attracted by the faster build time and good insulation properties of SIPs and would appreciate any thoughts/ experience.

Also would appreciate any pointers on how the build cost compares to standard brick and block construction.

narmada Sun 26-Jun-11 15:16:09

Should have said, we're in greater London also - so labour costs v high and keen for shortest build time possible therefore.

MissMarjoribanks Sun 26-Jun-11 15:44:14

If it's at the rear and not visible from the public realm planners don't usually care what it's made of or looks like. I've granted permission for lots of unconventional extensions at the rear. So you're unlikely to run into problems there.

If it looks crap and out of keeping though, it may well put buyers off. Some pre-fab materials are great, some are rubbish. The rubbish ones will weather really badly.

I would also talk to Building Control (not planning, separate department) at your Council as you will need to get their approval as well. They will be the ones who are checking it stays standing up OK.

Can't help on costings though.

Pendeen Mon 27-Jun-11 15:13:26

You don't say what is the construction of the house. Is is a framed and clad property? If so then a carefully chosen and sympathetically designed similarly constructed extension would probably fit in well, but if the house is of more traditional construction then the aesthetic challenge would be far greater. I am not saying such a solution would necessarily be wrong, the construction time (and sometimes) economic advantages are obviously attractive but it's a big decision and looking at the longer term is also important.

One thought: most SIPs have very little 'thermal mass' which means the interior of the building (or in your case the extension) will heat up more rapidly but also can become uncomfortably hot during the summer, whereas a traditional masonry construction acts to absorb and release heat in a more even manner.

Detailing the junction between existing and new construction for these typs of situations can often be tricky.

As for maintaining or improving the value of the property, I'm an not a valuer so it would be wise to chat to one or two local agents before finally deciding on the best solution.

Good luck!

narmada Mon 27-Jun-11 16:11:41

mrsmarjoriebanks and pendeen thanks so much for taking the time to reply - good points you both make. I hadn't considered the issue of overheating in the summer - the rooms will be NE facing, but I guess will still get hot on hot days.

The house is of standard construction - brick and block - perhaps we will just have to suck up the longer construction time but we have two kids aged 8 months and 3, so the prospect doesn't fill me with joy smile

lalalonglegs Mon 27-Jun-11 16:22:34

SIPS are super-insulated and, while very thin, give much better thermal values than conventional brick walls so over-heating/heat loss should not be a problem.

oricella Mon 27-Jun-11 16:34:50

I wouldn't worry about overheating either; we're building an entire house out of a type of SIPS and with superinsulation it's not the structure that will let in the heat. As for longevity - timberframe building is the norm up here in Scotland and I certainly would expect our new house to last; once rendered (often with blockwork) or clad they are not easily distinguished from conventional builds. You can also look out for BRE certification - some SIPS will have these, others won't

Pendeen Mon 27-Jun-11 17:04:58


Good thermal values ("super insulated"!! )are a measure of how efficient the structure is at retaining heat or to put it another way reducing heat loss which is not the same as preventing overheating which is partly a function of thermal mass. They are two quite different concepts.


I would worry about overheating in the scenario described by the OP.

In the situation you mention i.e. using rendered blockwork as an external cladding, the mass of the render and the concrete blocks would act as the thermal mass which is why you would not suffer from overheating, even with timber frame as the structural form.

narmada Wed 29-Jun-11 20:53:42

Thanks again everyone. Am I right in thinking that if you had rendered blockwork as the external cladding, this would add to the build time, therefore reducing the time advantages of using SIPS in the first place? As you can tell, I don't know much about construction!

Pendeen Thu 30-Jun-11 14:21:30


Yes - that is quite correct.

The use of masonry as an external cladding for timber frame, whether rendered or not, is often a planning requirement or (and here I am guessing somewhat because I don't practice in Scotland) a requirement of Scottish building regulations, probably because of their weather conditions especially in exposed locations.

SIPs or other timber frame has not really caught on south of the border,;much of the reluctance is driven by conservatism in the housebuilding industry and of the public in general.

Horses for courses really.

Here's hoping you have a successful project!

MothershipG Mon 12-Aug-13 19:22:55

Sorry to resurrect a zombie thread but 2 years on I'm wondering if the OP went ahead?

Or has anyone else done a pre-fab extension?

Has anyone used the Home extension company?

I need a box on the back of the house as cheaply as possible and I'm attracted by the speed and high insulation properties of this type of construction.

Any fresh opinions?

MothershipG Tue 13-Aug-13 19:53:23

No one?

deborah398 Thu 13-Nov-14 21:03:33

Hi MothershipG

Just wondered if you used the Home Extension company in the end as we are thinking about using this company.

Any advice would be really beneficial.

Kindest regards


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