A few things to think about;
1 The architect's offer to refund his fees- has he given you that in writing? If so then you can (more or less) bank it, if not then that offer will get whisked off the table soon as the summons hits his desk (though he has no defence against your claim of negligence so you're on good grounds there)
2 The £7200 extra- can you break that down into unavoidable costs (ie what had to be done to make the scheme work with that half brick wall) and avoidable costs (for instance if you paid £1000 for the original steelwork which turned out to be useless and you had to spend another £1500 on steelwork that was actually required for the scheme then you have a legitimate claim for the £1000-worth of steel (which wouldn't have been ordered if the drawing had been correct).
3 Likewise with time charges from the builder but again you wouldn't be entitled to all his additional costs. So say builder had to rip out 2 days work, which took him a day- you have a legit claim for those 3 days (that work would not have happened if the drawing had been accurate) but not for the 2 days the builder took to install the proper solution.
4 Any professional fees you paid to others for the design rectification- yes you can claim for them. Your original architect had the opportunity to redesign the scheme and didn't, effectively abandoning you.
5 Has there been any betterment within the scheme changes (for instance have you ended up with more floorspace or more headroom or any other quantifiable gain)- if so then you shouldn't really be claiming for that element.
Starting work before Building Control approved the plans was unwise but I don't think in your case it is a showstopper- you said your original plans were validated by Building Control, it was only on a subsequent site visit that they spotted the discrepancy. So if that's his only defence he's in trouble (and if he's local his next 10 jobs will be a mare if the client then uses LABC)
noseall has a point about that catchall clause (builder to confirm all measurements) but this is small claims, a layman (such as myself) would consider 'I've paid good money to some professionals to measure up and draw this scheme, why buy a dog and bark myself?'. If they'd drawn a wall that didn't exist or omitted a wall that did exist then it would be more reasonable for them to assert that you or the builder should have spotted the discrepancy.
I think you've got a decent case.
You MUST prepare a clear and honest timeline of what happened on what dates. You'll also need a chat with your builder about the avoidable/unavoidable elements (my guess is if the drawing had been correct your scheme would probably have cost £3000 ish more than your original price so you'd be looking to collect £4200 plus matey's fees.
OR do the timeline, claim for the full sum and see what happens- I reckon the fees are a given, a judge might well go 50/50 (cos of that clause). Don't worry about needing expert witnesses and stuff like that, the whole point of small claims is to adjudicate in cases where the facts are fairly clear and to keep the costs to all parties down.
Hope that helps, hopefully you're enjoying your loft
