Remember that you may also need a building warrant if you are knocking down walls. And you need that BEFORE you start the work in Scotland. It’s not like down south.
And it’s not as if you can hire your contractors on day one when you get the keys . First You need to engage an architect / technician / whoever is advising you on the reconfiguration of the property and get plans drawn up.
Unless it’s a very very simple job it’s worth paying a bit extra to get advice on how to use the space better . Builders will tell you if you can knock down a wall. They are not experts at looking at eg the whole ground floor of a house and working out how it can best meet your families needs.
If you are taking away walls You also need to Hire a structural engineer / other approved certifier and submit plans to council for BW.
Then you can get quotes for the building work involved . As a PP said, they won’t guarantee the price of a quote for anymore than a few weeks due to increasing prices.
Then you need to Engage your preferred contractor and get an estimated start date.
You can’t really get the plumber, joiner, plastered and electrician until you have a date for the building work to start. Any quotes they give you will be very short term.
Theese trades all know that your builder won’t start on time, let alone finish on time. So they are not going to block out two weeks next March to decorate your house.
So unless you have quite a lot of experience, I wouldn’t advise managing this yourselves. Better to get a bigger contractor who will do the whole job and manage it.
It’s tough at the best of time but even harder now due to shortages of materials and trades people.
It’s much harder if you are living there and you have to keep some sort of functional bathroom and cooking facilities. Also water, power, heating and drainage need to stay on or be reconnected each night.