What are the pros and cons of a phased retrofit?

Transitioning a 1950s semi-detached home into a modern, energy-efficient space is a marathon, not a sprint — but there are some elements such as rewiring where it's better to get the job done in one go...

Brigid Browne: ‘The 1950s-built house likely needs rewiring; over the years the house will have received changes here and there. When renovating a house of this age, you are often best to have the whole wiring system changed out. You don’t want to have to open the place a few years later and risk damaging your finishes.’

Dear Brigid
I am hoping to do a small house extension and have a budget that I thought would more than cover it.
I had hoped to do some retrofitting along the way, but having costed the project, it looks like the budget will not stretch to that.
What do you advise — should I focus on making sure the extension is well insulated and return to retrofitting the main house (a 1950s standard three-bed semi-d) when I can afford it.
Is it possible to retrofit on a phased basis or does it all have to be done in one go?

Yours, Philip