I think it would be better to define exactly what you mean by “3-bed semi”. They are not all the same, some are easy some take longer.
EG both of these are 3-bed semis:
New build:
Other:
I would charge quite a bit more for the bottom one.
Top picture. No flat roof straight up and down with a side window
I quoted a house in Croydon Monday. £20
Easily got it ,6 weekly last guy hasn't been seen for 2 years his price was £12 on ladders.
Bottom pic
Without a flat roof round back have one like this already £25 6 weekly
With a flat roof and bio doors £30 £35. Kingston upon Thames 6 weekly
I have a same style house as the bottom one in Wandsworth. £40. 6 weekly.
Charge what you want I do I have not lost a customer in 5 years. Only ones have moved out or poped there' cloggs