We have just built an extension to our house with wooden floorboards
as flooring.  There is a 1/2 metre of space between the floor and the
concrete underneath, and we want to build a trapdoor into the floor
and use the space for storage (e.g., wine).  I am looking for a DIY
building kit for installing a trapdoor, complete with hinges, edging,
cabin hook to lift it up, etc.  I want to use the wood that we cut out
from the floor to make the trapdoor, so that all the wood matches. 
The kit needs to be able to be purchased within the UK, ideally within
London.  I thought this would be easy to find but we have just done
the rounds of DIY stores and none appeared to stock them.  I'm
guessing it is something a boat or caravan maker might provide, but I
don't know how to find one.

Greetings!

I have enjoyed researching this question for you. 

This type of product has many names, trap doors, floor doors, floor
hatches, and access doors, to name a few.

Most of the trap door kit information, available online, involves the
use of ladders or vaults.  I did run across a company that may be able
to provide the materials you are looking for concerning your project.

You can peruse the products by visiting The Howe Green Company:
http://www.howegreen.co.uk/

This webpage may address your specific needs: 
http://www.howegreen.co.uk/productseries-special-adaptations.php?product=special-adaptations

Google Search terms used:
floor hatches+installation+uk

I hope this information will be of assistance.  Thank you for using
Google Answers.

madsky101-ga

Have you read that? 
http://www.dooyoo.co.uk/review/236729.html

Hello!

First off, I'd like to note that the folks in the UK sure seem to have
a desire to build in trap doors :)   Most of the sites found were
under the UK extension.

While the first link I found isn't a kit per se, it is pretty detailed
instructions on how to build in a trap door in your house, similar
(and for the same reason) to the one you wish to build.  The article
can be found here.

http://www.dooyoo.co.uk/review/236729.html (and was already listed
below!)

These folks sell DIY kits for trapdoors, but in the attic direction.  
You might contact them (as they sell in the UK) about a kit that would
work, sans the staircase.

http://www.stira.co.uk/

Unfortunately no one seems to have much on trapdoor building these
days.  Lots of articles on refininshing wood floors, or replacing
parts of panels, but very little on the DIY route.  Hope the second
link helps a bit

There are no kits as such. This is a carpenter's job. I have done
similar jobs for safes and gun cabinets here in the UK.

Your biggest problem will be supporting the floor joists [ often at
300 to 450mm centers ] you will have to cut. You will need to fit a
trimmer [ or a carlin, to give it it's correct name ] to support the
ends of the cut joists and transfer the loads to the continuous joists
each side of the opening. The depth of the trimmer should be 25% more
than the joist if you are cutting out more than one joist.

Cut your floor boards with a fine tooth jig saw between the joists and
with a panel saw over a joist. Use a new, sharp blade and a skilled,
steady hand for a perfectly straight line. For a perfect fit, allow
for the edges of the hatch to be trimmed with a beading.

The floor boards you cut out will need to be fixed to a frame to hold
them together and form a hatch. The hatch could just drop in or be
hinged. If hinged, fit a pair of solid brass hinges using brass screws
of the correct gauge and length to suit the hinges. Finish securing
the screws with all the slots aligned. If the barrel of the hinge is
set below the level of the finished floor [ ie. you will not feel it
with your toes ] the hatch will not be able to open out fully flat,
just a bit over vertical. The hatch will need a stay fitted to stop
the hatch opening too far and breaking the hinges. The hatch should be
supported by a stop [ frame ] all round, fixed to the joists/trimmers,
and not rely on the hinges for support.

Let in a brass flush ring pull for lifting the hatch. These can be
obtained from yacht chandlers or some good merchants like Jewsons.
Alternatively, fit a surface mount cabinet lock under and use the key
for lifting the hatch.

The sub-floor may well be just earth, hardcore or rough concrete so
allow for the installation of a timber platform to take your wine
racks. Use only vac-vac preservative treated timber for the platform
and lay it on a damp proof course.

If you need a light other than a torch to see your wine labels then
use a switch with a neon 'on' warning light within the room so you
don't forget to switch it off. Getting lighting circuit power to the
light may be a big problem as your lighting circuit will normally be
in the ceiling above. A 5 amp fused spur off a power circuit at
skirting level is proberbly the best bet.

If you are into DIY and have all the tools already then it is a
straight forward job. If you have to spend £500 to buy the tools then
maybe a call to your friendly chippie is the way to go.

Good luck.

lugga

Thanks everyone for the comments, much appreciated and very helpful. 
No, I hadn't seen that dooyoo link, I can't believe I missed finding
it in my own searches.  I think the reason we are so into trapdoors in
the UK, especially London, is that our homes are so small by US
standards that any extra space we can squeeze out we do!  My husband
isn't a carpenter but he's got all the tools and has muddled his way
through things before OK, so I think with all the advice here he
should be able to do it.  Thanks again.  :-)

This page will allow you to view access doors suited for pedestrain traffic:
http://www.howegreen.co.uk/productseries-details.php?product=3500sc