The definition of building coverage ratio is not what you suggest. Instead, it is the floor area of a building divided by the area of the site. http://www.realestate-tokyo.com/news/floor-area-ratio-and-building-coverage-ratio/

Nevertheless, BCR relates to density. Let us suppose that the price of land is lower in rural areas than it is in urban areas. Then, it makes sense to develop land at lower densities in rural areas than in urban areas (this does not mean fewer building per unit of land necessarily - but fewer tall buildings), because it makes sense to economize on valuable land. So BCR is a function of land price.

Peter is right that BCR or FAR is a function of land price, but land price is also a function of FAR.  The causal relationship goes in both directions.

BCR or FAR is an important variable to distinguish urban area from rural area.  However, caution is necessary when trying to find a threshold value for that purpose.  First, as a result of suburbanization or urban sprawl, the boundary of urban area and rural area has become very fuzzy and difficult to identify.  It becomes then questionable if a threshold value of BCR or FAR can really distinguish urban area from rural area.  Second, as is well studied in urban geography or urban economics, many other factors need to be considered when distinguishing urban area from rural area.  Examples include continuity, commuting pattern and occupation.  Even if a threshold value could be found, it is doubtful if that would be sufficient to define an urban area.

Hope this helps.  Best,

FD

These ratios exist with and without public sector controls. The government can set a maximum BCR. You will observe BCRs that may be below or above the current maximum BCR because of choice (you can build with less than the maximum) and differences between current and previous maximum BCRs. The value of land does depend to some degree upon the maximum BCR. But just because there are constraints such as this, it does not mean that those constraints are binding. You might be interested in the following article:

https://www.researchgate.net/publication/5141967_A_Note_on_the_Optimal_Design_of_an_Office_Building

Article A Note on the Optimal Design of an Office Building

Thank you both for your answers. I was aware of them but is nice and helpful to have a further feedback from you. Actually the question was also on building density (n°of buildings/hectare) but i think that the answer is the same given for BCR.

I don't have a number for you, but I did a project once where we wanted to define the boundary of urban areas (in the UK) from maps. It was difficult. We thought that it might be easier with larger scale maps (ie. 1:5000 rather than 1:50,000), but it turned out to be harder. With smaller scale maps the density gradient was more clearly evident and we were able to draw boundaries based on visual judgment. The objective was to quantify the rate of urban expansion over time, for which our approximate method was adequate. It might have been interesting to go back and check the building density values at the boundaries that we drew - but we didn't do this.

A measure that I have seen is the % of land area devoted to roads - this tends to increase with urban density. It might be easier to measure from maps than building floor areas for which you need individual building footprints plus the number of floors which are not evident on maps.

Hi Carlo, this is an interesting question but the answer will depend on what exactly are you after: regional analysis of urbanisation based on satellite imagery? Correlation of building types with density measures? Some kind of urban planning controls?

Be aware that is common in the literature to conflate various building density and population density measures. The 'urban' vs 'rural' is a socio-spatial distinction, so it cannot be reduced building density alone. The below publications may help.

Article The urban density assemblage: Modelling multiple measures

Conference Paper Nothing Gained by only Counting Dwellings per Hectare: A hun...

I don't think BCR or FAR can define urban/ rural areas; there are so many compact and especially spread ''cities'' all over the world; urban areas with very low density...

Otherwise, in some countries, areas are considered ''rural'' areas if they are agglomerations of less than five thousand inhabitants, in which the distances between the buildings can reach 200m.

Dear Carlo,

I believe you should go to the basic and that is how you define urban area from rural. In rural areas one can not find public programs for general public, beside elementary schools  and kindergartens. Start from that fact number is irrelevant.

Ti saluto

Višnja Kukoč