Sort of. Don't care about radiators or bathrooms but do need the total gas input power of your cooker. You've also not said what size combi you're thinking of for the outbuilding
A 24watt would be more than sufficient for the outhouse...
As for the cooker i have no idea but if you could base it on a average cooker of that size would that make it easier?
We may change the cooker soon anyway so would be good over spec a little for future proof
OK, as a rough estimate I'll say your cooker is 15kW. That makes a total load of 51kW in the house (assuming you've not forgotten to tell us about any gas fires/tumble dryers etc), which for starters means that your 22mm gas pipe in the house is almost certainly undersized and will need upgrading from the meter to at least the point where it tees off to the first appliance before any other work is done.
Realistically the most you can pull through a standard domestic gas meter is 65kW, but the smallest available combi boiler is 24kW. 51+24=75, so your gas meter will need upgrading. The bigger gas meters are quite chunky - 180 mm X 395 mm X 280 mm - so won't fit in a standard meter box. You'll therefore need your meter box changing.
As you're busting through the 70kW domestic gas engineers' legal limit, you'll need a commercial gas engineer to do at least some of the work.
As a sight-unseen guesstimate, you should expect to spend in the region of £4k getting all the gas prep work done before you purchase the new boiler and have it installed.
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