Some things just depend on what you want but here is a fairly mid range budget for fixtures. You can spend loads more on a kitchen / flooring / lighting...
Single storey Extension is 1,500 psm so 15k. Add to that flooring (3k), light fixtures (1k) kitchen, roof lights (2k), under floor heating (2 k), loo uner the stairs (2k), kitchen including appliances (15k for mid range), new boiler (2k), bifold doors (5k), fees including planning, architect, building control, Thames water, Partywall surveyor, measurement survey (10k). Canso in total a mid range extension is circa 57k. It can be for less or a lot more. For the utility room I am just assuming washer / dryer sink and a few units so add 2k maybe.
Depending on size a standard dormer loft with ensuite Including the cost of new bathrom suite will be between 40-45k. If you don't have to redo everything it should be less than that.
Timber sash windows are circa 1.5k each though can be as much as 2.5k for certain companies. A bay counts as 3 windows so you have 11 in total. Budget 16.5k though could be more!
The cost to redecorate will depend on the size of house and what needs doing. Are you replacing all the bathrooms (how many), all flooring, skirting, doors? Do you need to strip wall paper and therefore potentially repaint and replaster. What about the garden? After an extension your garden will need work anyhow but is everything else okay or will you need a new patio, fencing? Are there levels?
We recently did a gut refurb of a small terraced house and had to do all of the above. Our spec was higher than quoted above and we did a double loft conversion and turned one bedroom into a walk in closet and ensuite. Also we slightly extended the first floor bedroom, laid a Victorian front path, etc. all in, it's cost circa 230k!