I'm not a fanatical DIY'er but do my fair share of painting and tending to my garden.
Firstly Durham has a HUGE B&Q in the retail park at Belmont which stocks everything you could possibly need for DIY. It has plenty of free parking spaces and is open usually 8am-10pm weekdays and also open on weekends. It stocks a large plant selection, bathrooms, kitchens, anything you can think of. I have never found any items out of stock either. It often has promotions on, or earn cashback if you purchase online eg grom topcashback.co.uk
It's easy to get to from the City Centre, with many buses going direct from the City as well as from nearby villages. If coming from further a field take the Durham turning from the A1 onto the A690 and take the first sliproad to Belmont. At the top of the slip road take a left, go over some traffic lights then at the mini roundabout take a right and you can't miss it.
Another large DIY shop is Honebase situated at the Arnison Centre retail park. It is smaller than B &Q but has an upstairs full of bathroom/kitchen/bedroom ideas, has a plant area and your usual DIY items. It often has 15% off everything days and you can collect nectar points too.
Easy accessible and situated on A167. If coming from Motorway either take Chester-le-street or durham turning and head for A167. Free parking available, may have to park nearer to another shop as gets busy.
Both are easy to find, everything is set out well instore, B&Q has large signs on each aisle, store workers are usualluy helpful.
Local DIY stores are available in small villages but usually have only a small stock and limited opening hours. Find on yell.com.