If you factor in half the time sitting round doing not much you should be able to charge your self a nominal fee if you must , where as Mr or mrs shed builder has rates, pensions,wages, materials and vat to add to the lot. Ohhh yea and they are running a business and aiming for min 30% profit and they have to pay tax, which is also factored into the profit req. You can't replace time but it's time well spent. If of course you earn £100 an hour then go to work and pay for it. "Off the peg" sheds a-la B&Q special are cheap but phenomenally shit. Say you pick one up for a few hunge, that's 3-4 hours worth of time you're giving yourself to build a one-off and that's before you buy any materials. With that amount of time you would have just finished the design!