Garden soil need to be replaced?

I flip my soil before winter.

After the snow melts and the garden is soft ... I flip again.

2 weeks prior to planting ... I dig out enough soil for 4 bags of triple mix and 2 bags of cattle manure.

The soil that I dig out ... with all its nutrients ... I toss on to the grass ...

I do this annually regardless

Although natural ash is generally beneficial to the garden, not sure about ash from a house with varying types of materials included in there. Personally I would change a few inches

I wouldn't plant food for a year. Flowers would be ok

If it burned down during the winter can’t you scrape the dirty snow off and dispose of it. How much ash was there? Inches of just a few flakes?

Unless we’re talking inches at best I’d just scrape off the top inch or so, worst I’d just plant like normal.

Realistically, it depends on the level of contamination in the ashes and how much of it leached into the soil and how far it got into the soil. Given the fact Winter snow and rain basically run pretty deep into the soil as it melts, I doubt that the first few inches would be enough if you are afraid of contamination. I would think that any contamination would go much deeper than you are thinking. However, that may also mean that the concentration of the contamination is also lower than what you might think as well.

If you are so concerned, the best bet may be to remove the root depth of soil - ie whatever you plan to plant, remove as much soil as needed so that the plant roots don't touch any potentially contaminated soil.