Phil L wrote:
John wrote:
We have a single storey extension about 7m x 4m and were wondering
about making it two storey.

How would we find out if the foundations were adequate?

Dig a hole down the side of the wall until you find the concrete it's
sitting on, then dig down the side of that until you find the bottom, the
BCO will insist on these test holes prior to any work commencing and you
will probably need them before plans are submitted, which means they could
be left open for a long while, or filled in with soft material like sand for
easier removal next time, you only really need one hole but two won't do any
harm.

If not what options are there?

You either live with the extension you have, or knock it down and start
again, underpinning will cost more than the last war, but as long as the
extension is on at least 250mm of concrete and the concrete is on a firm sub
base like clay or stone, then it's 90% certain that it'll be ok to go
ahead....it may even be on a raft, which again will probably be OK.


What are the "order of magnitude" costs for either putting on a second
level and extending the pitched roof, or rebuilding from ground
upwards?

Any thoughts welcome!

knocking down and rebuilding is obviously the most expensive - new
extensions start in the region of £600 - £800 per square metre (outside
measurements - walls included!), but demolition will set you back a few
grand too, probably around 5-6K.
So a new double storey would set you back:
7.5m X 4.5 metres = 33.75m2 + 33.75m2 (upstairs floor area) =
67.5m2....times this by £700 and add on 5K for demolition, grand total =
£52,250 is probably about the lowest price you'll get.

The price for extending upwards only (provided foundations are OK) is much
less, because A) there's no digging or concreting involved or any associated
groundworks (relocating drains, manholes, water, gas and electricity mains
etc)
B) some of the existing roof timbers may be used as joists for the upper
floor or provide support to new ones.
C) There's no concrete floor to lay - this is a major factor in many
extensions and accounts for about 10% of the total cost once excavation,
insulation and screed is factored in.
D) it's much easier!

It works out around £400 per m2, so 33.75 X £400 = £13,500, ish....as you
can see there's a difference of about 40K, so you'd better hope and pray
that the foundations are good!!


I disagree. Demolition and groundwork were the smallest part of my
rebuild here.

The time is all spent in te fiddly bits and the money is all spent on
the fittings.