Extension Build Thread

9 Jan 2015
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United Kingdom
Hello all,

As mentioned before, I have a lean too that is in severe disrepair, and am slowly working towards submitting my planning application for building a new extension in its place.


The Grey extension pictured above is one possibility. An alternative, is the version below:

Alternative View from above joining neighbours garden:

In the apex roof version (Version 1), the glass pictured would be either Aluminium Bifold, or a 3 track sliding door.

My question is:
Is it a bad idea to have the centre valley of the roof (and hence much of the weight) in the centre of a bifold or sliding door?

I assume I would need a considerable amount of structural reinforcement (RSJ) over the top of the glass, but is this likely prohibitively expensive? Is there a better way to do this?
For reference, the extension is 8m wide, and the glass expanse is 6m wide x 2.1m high.

Any suggestions are welcome!
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If you consider that the apex roof will divide the load to two line loads on the side walls and two point loads above the bifold doors and below your existing first floor windows, you will find that the loads upon your bifold door lintel and rear knock through steelwork are actually pretty similar when the additional weight of tiles is taken into account.

Its a light load in RSJ terms and in-fact additional weight is more likely to help restrain a lintel or rsj to prevent movement caused by swinging and slamming sliding or bifold doors, which is a common issue.

At a 6m span, an RSJ would be my choice for minimising bifold / sliding door issues.

Hence decide what you want in regards to skylights or laterns and go from there as the difference in cost of these items is far more than the additional cost of some upsized steelwork.
Thanks THughes! :)

I think I prefer the look of the Apex roof version, so this is good news + I am concerned that the flat roof version would not be seen as valuable to the mortgage valuers. Although the house is not an investment as such (bought is as we started a family) The recession was a painful time for me, so I am mindful of keeping a good LTV ratio in my home!


The sliding door pictured comes to £11600 inc VAT (current best quote for aluminium) @ 6150mm x 2100mm opening. OUCH! I think its the size of the glass that causes it as I can get a Schuco aluminium 6 leaf Bifold for £7200 (schuco) or about £5700 from VUFold. I would prefer a sliding door, so if anyone knows of a better deal, let me know.

Below is a plan view showing where I propose to put the foundations and the steels.

Foundations in Green
Steels in Orange.


The reason for the steel on the left side is that my neighbour has built on the land boundary, and I wish to build up to it also, but don't want the complication of building a long wall partially on his foundations, which are on my land.

I don't know if the blue line would be steel or wood. Ideally I would like it to be an Oak beam (for aesthetic reasons), but have been told this may be a problem for an engineer to spec the size, as the strength is dependant on the individual beam.

Steels 2:
As there is so much steel already, should I/could I build the entire structure in steel? My background is in wood, and I have never done any brickwork. I'm happy to learn, but if the price is similar, and it will be easier to do this in bad weather (as it looks like I will be doing this ASAP) It would be better for me to decide earlier rather than later.


I have added a small veranda. at the front (the roof overhangs the supporting wall with the windows in)

As the foundations are level with my joining neighbour, is this likely a bad move where the planning office is concerned?
Last edited:
We are building our extension at the minute and we have a 3 pane sliding aluminium door going in (c.6.3m wide and 2.1high) and the cost is less than 60% of your quote. Window company have a good reputation in our area and have been very good in helping plan etc. It will be worth shopping around.
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I will sort out an update shortly! Buck 68- I have since managed to get much better prices on the glass, thanks to your prompting, thanks!

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