Estimating the cost of your build

How much does it cost to build your own garden room?

Those of you with a good memory may recall that my original estimates on the build costs were around £6,000-£7,000, which after a bit of cursory research I imagined being spent like this:

  • SIP panels and sub-frame = 3300
  • plinths (footings) = 700
  • cladding = 800
  • window and door = 900
  • miscellaneous (plasterboard, fixings , lights, roof covering etc, guttering , etc) = 1000

TOTAL £6700

This was a pretty encouraging figure and enough to get me started on the project, but unsurprisingly it did not take long to start uncovering costs I had not considered.

My latest estimates is £8000. These were the main decisions that influenced the total costs:


I decided not to skimp on the SIPS and went for thicker than strictly necessary panels (125mm) on the walls and floor. Since I will not actually be living in the shed, the normal argument about long term energy cost savings does not really apply, but it is what I did anyway.

The panels, fixings, connecting timbers, breather membrane and sub-frame represent about 50% of the costs. Could this part have been done cheaper? Well, by far the cheapest approach is to build a log cabin and then to insulate it – which I think would easily halve the build costs – but that would be cheating.

It is possible to cut SIP panels yourself and avoid the costs the SIP maker incurs doing the manufacturing drawings, setting up the machines etc. I looked at this briefly, but it comes with pros and cons:


You would need to buy one of these to cut the panels:

beam cutter

.. which looks like fun.


you also need one of these to cut the rebates in the EPS foam:

hot knife

…and they seem only to be available in the US.

My guess is that unless you were doing a much bigger building, the cost of the tools (if you could get hold of them) ; wastage from using standard 2.4×1.2m panels; plus all the expensive mistakes you would make doing it for the first time round would probably outweigh any savings.

Window and Doors

All the top-end garden buildings come with aluminum doors/windows, but the absolutely cheapest I could find a 1m window and 1.7 m sliding door was £1950 (supply only).

I am pretty sure this is a great price since the shop was recommended by a Geordie at work who is having some building work done. He is a self confessed skinflint, and the original recommendation is from his builder who he assures me is tighter than he is.

Anyhow, aluminum was way too expensive for my budget so I have to go with upvc. The same shop will supply window and door for a much more wallet friendly £890. The doors are manufactured by Eurocell, who are one of the few manufacturers selling in the UK who supply grey upvc facias.

As an aside, my door research took me briefly into the rather disturbing world of ‘green building’ . This is not a topic for the faint hearted, but I managed to stay engaged long enough to discover that the only remotely affordable super-eco door/window manufacturing firm is ‘Russell Timber Technology’. This firm, as the name suggests, makes timber frame doors. The prices are still eye poppingly expensive though.


well I already covered this – not only am I going with an expensive ‘mini plinth’ option for fear of sinkage, but frankly I bottled it at the last moment and ordered 11 rather than 9 mini plinths – cost £750

Could this be done cheaper? Yes, stick it on some railway sleepers.



  • The spot I am going to be building on is now clear and reasonably level (see pic below)
  • I approved the SIP manufacturing drawings at the start of the week and the panels should arrive mid December
  • Doors, windows and plinths on order
(almost) level

UPDATE – 2016 – interested in what it actually cost in the end?    Take a look here :

Leave a Comment