It isn't clear if your original agreement between friends was for him to draw up both sets of plans. As this is his job he would know that there were two stages to this - did you know from the outset that the plans for permission would not meet the same standard required for building regulations? If so, did you discuss this with him?
Another answer suggests that there is a difference between your "friendly relationship" with your friend and an "essentially business relationship", but to be honest I don't see that any business relationship exists between you at all. He didn't offer you a freebie as a business arrangement, he offered it to you as a friend.
Consider this example - let's say a friend asks you to give him a lift getting a piece of furniture out of his house, and you agree to this favour. After you have helped him get this heavy item down his stairs and out the front door you find there is a van waiting and he now expects you to drive this van across town to another house to deliver and install the furniture for somebody else. You'd feel a bit put out, right? What you are being asked to do goes beyond the scope of the original favour. Yet if this hypothetical friend had called a removal company and made a business arrangement the details would have been discussed at length, priced and agreed upon. A business arrangement and a favour are very different situations.
Your friend is an architect, that is how he makes his money, so he doesn't work for free. If he drew the plans for you in work time, he lost money; and if he did it in his own time then he lost time with his family, on his recreation time, whatever. Taking what you wrote literally, he "kindly offered to draw up the plans for a house extension for free", and he did just that. If what you now expect him to do to finish the job was not discussed originally then you need to approach him to ask a new favour, not simply expect it.
See it from his point of view - if you are expecting him to do something he didn't originally offer then he may feel like he is being taken advantage of; yet this does not mean he is not willing to do you another favour if you ask! Friends do keep doing things for one another, but nobody likes to be taken advantage of. So, if you are still hoping for another freebie perhaps you should humbly ask:
I hope I am not asking more than you originally offered to do for me - please tell me honestly if I am - but I am at the stage where I need plans for building regulations and I was wondering if you could do those for me? If it is too much work to do for free then I am willing to pay, as I did offer to pay from the outset. I don't want to cause difficulty for you, I just want to move my project along.
On the other hand, if you are absolutely certain that his original offer included him preparing two sets of plans for the two stages of planning then I still think you should approach him in a humble, rather than an indignant manner, because there may be good reasons why he can no longer do this for you, for example, if his spare time or work commitments have changed in some way. Perhaps say:
I'm sorry to mention this, but my building project is held up at the moment waiting for plans. I really appreciated you offering to do them. If it has become a burden to you I understand. If you'd like me to pay for the work please let me know the cost, or if your time won't allow then please let me know so I can find someone else to do them. I don't want to cause difficulty for you.
