I'm not entirely sure what the problem is.

If you have a base map, you can add layers on to it, and place whatever it is you want on those layers without affecting your base picture.

of course the layers must be transparent, so you can see through to your base map with the only opaque thing being the added content such as the building.

For starters try selecting just the part you want to add, to the map such as the building using the selector tool in point to point mode. this will let you create a tight selection around the object you want.

you can then copy paste the object as a new layer. Using the Paste as new layer option, or holding down control +L keys.

with that done, you can move around the layer and change its colors, size etc.. without affecting your base map.  

You can keep adding layers, and hiding or showing them all without affecting the base.
 

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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.  

Thanks, vacunita, for your speedy reply. I think this may be the answer although I am still uncertain about one point, which I will mention later.

Apologies for a poor description of my problem. After posting it I thought - I could have done better than that!

Here is a revised version of the original question in case it helps:

I have to reproduce four drawn maps, basically identical, except that each shows different developments of a few buildings, each some ten years apart. The artist has drawn the original basic map four times so that each one is slightly different and contains different drawing faults.

For reasons of economy and standardisation, I have produced one 'good' map which I would like to use for all four, but cannot see how to add the small modifications by scanning them and pasting to their respective basic map.

I realise that your suggestion of putting the revised information on layers may be the answer but are you saying that I can copy and paste new information to these layers from a different scan?

Eric
 

Sure, if you have these scans opened in PSP, you can use the selector tool, to select pieces of the map you want and then paste them as a new layer onto your new basic map.

Imagine you are working with overhead projector style transparencies.

You can cut a piece of one transparency and overlay it on top of another one, and since they are transparent your first transparency will be seen through the second one.
You could even add an opaque picture at the bottom to act as a background.

Same principle here, you cut a piece from one image, and paste it as a layer onto another image.

 

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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.  

Once again, many thanks for a very helpful and easily understood reply. I will certainly give it a go.

Eric