A professional home survey is a must
Believe it or not, there a tradesman out there who think it is acceptable for the customer to take measurements themselves. Whilst this may be tempting to save on a survey cost, or to save time if your tradesman is very busy, this is a big no no. By providing measurements yourself you are liable if anything goes wrong, and we have seen it happen too many times to unsuspecting customers who aren't sure about sills, thresholds, brick allowances etc. The sizes are the responsibility of whoever is installing and if they do not want to carry out this service walk away.