[finster]

What depth of topsoil do I need to put on a bed?  Does it need to be dug in or is it just a dressing?

[Runwell-Steve]

That is all dependant on what soil is there already, and what you are planning on growing as well.

I have never put top soil on a veg bed, as it works out very expensive,  I have just dug in loads of organic matter over the years, well rotted horse manure, garden compost etc..

What is it you are looking to achieve with the top soil.  Is it to raise the soil level in a raised bed, or for some other purpose.

Top soil cost about £40 a ton, in order to increase the height of a 10m x 10m plot by 10cm you would be looking to need about 10 ton which would be £400.00.  You could get loads of manure for free.