This is subjective. If the current basement is not extended to the full footprint of the house, then it would be easy, as all the expansion can be done in the crawlspace. However, if the basement extends to the full footprint of the house, we recommend you consider if you really need a wider basement. It is still possible to expand but it will cost a lot of money.
Foundation and roofing are two of the most expensive projects in a build. A basement extension will require both roofing and foundation, without the added square footage of an entire home build to reduce the overall cost of the new addition per square foot.