18 months.  From start to completion.  That’s the architect’s estimate.  It might be a bit shorter – say, 15 months – but not dramatically different from the 18 month estimate.

So how does this stack up?
Initial designs 8-12 weeks
Planning permission 8-10 weeks
Building regulations 8 weeks

So that is 6-9 months before we can even start on the building work.  With the building work estimated to take a further 6-9 months, you can see how an 18 month estimate starts to look like a reasonable guess.

Until the designs are complete, it is going to be difficult to come up with a more accurate estimate in terms of the timing as well as the costs.  I must admit that 18 months seems like an awfully long time.