Dissolving "neighbour"/bordering polygon features in QGIS

Question

I have a polygon layer with buildings for the area of the whole city.

My question is how to dissolve/aggregate/merge only those buildings, that are "neighbours", in a sense that they share at least one mutual wall or the distance between walls is miserable, e.g. roughly saying less than 0.5 m.

My desired result should look like

I have in mind several approaches which might be applied.

1. In layer create a column with a unique number and then apply

Vector > Geoprocessing Tools > Dissolve (by this unique number)

2. With much of manual work

Select Feature by Freehand > Merge Selected Features

Unfortunately, in both cases, it requires tremendous manual work, therefore I would love to optimize the process.

For large-scale maps, it is definitely an advantage to have each building separately, nevertheless, when working with bigger scales, the appearance of too many buildings on a small piece of a map, makes it overcrowded, causing the "ruffling effect" and cartographic generalization is a strong necessity.

The following examples do not really suit my request:

• Dissolve polygons by specific field- but only neighbours polygons
• Dissolve polygons - group to others with touching corners
• Merging overlapping polygons with Dissolve?
• Dissolve polygons in QGIS (GEOS) with tolerance

Answer 1

1. Add a 5m buffer, and choose the option to dissolve the result.
2. Remove the buffer by running the buffer tool with a -5m value buffer distance (negative five meters).
3. Then split multipart to singlepart.

If necessary, run a spatial join to get back individual building attributes.

Answer 2

I have found a way which optimally fulfils my issue.

1. Produce Buffers for each building set

   Vector > Geoprocessing Tools > Fixed distance buffer

   • Buffer radius was 0.5m
   • The Dissolve option was enabled

2. Perform a clip between the original layer with buildings and a new layer with buffers

   Vector > Geoprocessing Tools > Clip

3. Split a new layer

   Split selected multi-part features to a single part

   

   There is also a QGIS plugin called Multipart Split which could probably help.

4. Transfer all attributes for each feature from the original layer to a new layer

   Vector > Data Management Tools > Clip

Additionally, the Filter... "Area" > 150 could be applied to reduce the number of buildings with the elimination by area, those that are smaller than would be not visalized.

However, there still some additional work left, because of the small distance between buildings (more than the radius) and inconsistent geometry of the original layer with buildings.

Thank you guys @csk and @ThingumaBob for suggestions and inspiration.

References:

• How to merge single polygons to multi part polygons by maximum distances?
• Converting selected multipart features to single parts in QGIS
• Documentation “Join Attributes by Location” QGIS 2.18
• Join Attributes by Location - Precision