Is it better to extend an existing city or build a new one?

I'd say extending an existing city is usually more practical than building a new one from scratch. The main reason is infrastructure - when you expand a city like Shanghai, you can gradually connect new areas to the existing subway lines and utilities, which saves massive costs. For example, when they developed Pudong decades ago, they extended metro lines across the river rather than building everything new. That being said, completely new cities can sometimes work if they're planned as special economic zones with clear industries, like Shenzhen was in the 1980s. But generally, expansion feels more manageable because you're building on what already works.