I have managed to find a chart that gives a very comprehensive comparison and contrast between these two works of literature, and I have included a link to it below, along with links to the enotes study guide sections for these two great novels. It is important to remember though that Zadie Smith "re-wrote" Howard's End, but clearly modernising it for her generation, using the central concepts and ideas.
This is why if we look at both of these novels, the Belseys and the Kipps are compared explicitly with the Schlegels and the Wilcoxes. A member of the Belseys and the Schlegel family falls in love with a member of the Kipps and Wilcox clan, and feels liberated being out of their own family. An accidental acquisition of an object--be it an umbrella or a Discman--leads respectively Leonard Bast and Carl to enter the lives of the Schlegels and the Belseys. There are some of the plot parallels but I will leave you to peruse the chart and find the rest.
What is important to realise though is that both of these novels are not just simply linked by a similarity in plot, but in theme as well. Both novels are explicitly concerned with a larger moral question, which is how the rich should interact with the poor, and what can the rich do to really help them. Clearly, the two families in both books have very different ideas, and Carl and Leonard Bast become the guinea pigs that allow them to put their ideas into practice.