When it comes to detail, we like to leave a “blank canvas” so to speak, so that the client can “dress” their house to their tastes. However with an existing house, we assume that the client likes their house, and so we look for features to replicate or enhance, in any event, my assumption is that the style of their existing house is a starting point. We don’t like to impose any design features on a client, it’s almost a policy to try to describe as “pay homage”. This approach is particularly important when designing a facade. Sometimes you can create a frontage which is just too busy for instance, and then the effect can be overwhelming. On the other hand, extracting a detail and using that in some subtle way often works well.
Where a completely different approach is required such as where the existing house is devoid of any attractive features, and/or the client has made it clear they do not like the appearance of the house, then we do have to look for other design clues. I had better just add that the old adage “less is more” should be remembered, especially in a modern design context where you are relying on the whole form and shape of the building to create the chemistry.