Pre-Stobart, Southend was tiddly with very few commercial passengers flights, and an expectation by anyone buying a house near Southend that it would remain a very quiet airport.
It's been transformed into an airport capable of (and within a few years likely to deliver) 2 million passengers per year. Ten years ago, the expectation was that Southend would remain a small quiet landing strip with very few commercial services. Furthermore, given the past, it is likely that in 5 years time Southend airport might ask the relevant Govt body to permit more than 2 million passengers per year.
Anyone buying a house near an airport is thinking not just about noise now, but about likely noise in 10 years time. If the *expectation* by a house buyer is that noise will be significantly greater in 10 years time, then better not to buy the house at all and look at a different area. While the number of houses on the market remains static, demand falls - the result is that house prices either fall or do not rise as much as in surrounding areas.