My favorite part of The Ogre Downstairs is that when the dragons' teeth are sown in the parking lot, the resulting tough guys speak "Greek." Really, it's English rendered phonetically into Greek letters, so you can decode it. That and the last bit where they have a contest to find the most hideous possible thing in the house. It always gave me really strange ideas about what Britons must have hiding away in their cupboards.
Ogre Downstairs is another very early novel, early enough that DWJ was still having to put a gloss of Issue Novel on the story. (For those younger than I am, back in the 70s it was well-nigh impossible to get a children's or YA novel published if it didn't deal with an Issue like divorce or teen pregnancy or drug use. Thus Witch's Business and Ogre Downstairs have something of that flavor to them, partly out of sheer necessity.) It's all about a new step-family learning to live with each other, with the dubious help of a mischief-making magic chemistry set. There is a lot of very funny stuff in this story, though some of the jokes are either so British or so 70s that they are a little hard to get.
I just picked up a new-to-me copy with this cover on it to supplement my ancient copy that is falling apart. Pretty good cover, I think.