If you're worried about how potential extensions could affect your home in the future, take a look at the planning permission granted for this development - in recent times, many housing developments have a clause that prevents owners from extending their homes without applying for planning permission.
Generally, it is OK to add up to 40sqm to a home, subject to certain restrictions (known as Exempt Development), but due to higher density developments and smaller garden sizes, local authories have been adding a condition to planning permissions preventing this. If the planning permission for this development has this clause included, then you have the added security of knowing that no-one can extend to their home without you getting a chance to look at drawings and have your say, through the planning process.
Overall, a two storey extension can only be erected in the following circumstances:
- Exempt development (as discussed above) and if permitted, then the part of the extension which is at first floor level would have to be a minimum of 2m distance from your boundary;
- Planning Application - where you can make your views known (by submitting an observation) and where the issues of overshadowing and access to natural light are part of the amenity level of your home and will be taken into consideration by the planning department in making a decision.
My choice of the houses indicated would be either 21 or 25, with 25 maybe having the edge..
If No. 19 were to extend at 2 storey then the building line would most likely be along your shared boundary, whereas with 21 or 25, a potential extension would most likely be along the opposite boundary - due to the side pathways, this means that the shadow cast wouldn't fall as far into your garden, particularly in the evening.
Also no. 25 is at the end of the cul-de-sac, so less traffic going past the house. On the other hand, given the size of garden on No. 26, this is the property that probably has most scope to extend, so you should take into consideration that an extension along this boundary would create shadows in the early part of the day.
Hope that help - best of luck!