That would depend on its design and the new design. Often easier to start again. Only a builder looking at it could answer this question.

Roof now is a rectangle shape gable end roof and im wanting extending the rafters to cover the length change over the extended areas

can you share your original and new floor plans?.. it sounds like something we've done where the new roof covers original house and extension to make one new shape rather than a separate roof over the extension but doesn't necessarily impact old ceilings under a gablel or hip shaped roof but.. something to keep in mind is that when the roof is enlarged at the same angle as the original roof then the new ridge of the roof is higher and you may wish to consider options for the extra head height inside the new roof (we had plenty of space for a generous loft with pull down ladder)...it always depends on specific dimensions and angles but widening our roof to cover an extra 5m meant that the ridge of the roof was raised almost 2m and cost an extraordinary amount of money as it needs an experienced builder with a skilled team of carpenters and roofers who can remove the old tiles, hand cut new roofing timbers, install insulation and steel roofing within hours so that your roofless house isn't exposed to the weather for too long and that's why many people choose a simpler roof structure for extensions and verandahs

Thanks for that, that's the thing I don't have plans as yet so before we go down a path we curious to know how it could be done. Makes me think it may be cheaper to remove the whole old roof and replace with a new truss frame roof.

nothing will be cheap and trusses won't necessarily be better so start with a builder to consider options and there may be alternatives that you can only discover with a skilled professional to investigate what you already have...(and/or post your original house plans and a wish list of what you want to achieve and HOUZZERS may suggest new floorplans ... you mentioned ceilings and i'm now wondering if the extension will also remove original external walls to make new rooms that combine original and new rooms and that creates more issues that need structural decisions...lots to consider

The photo is what i have versus the red line of what i would like to do. The extension on the back is 4 metres wide so i want to coverthat with the new roof, its a skillion roof and go forward 2 metres to cover the extension and front verandah

Yes a new probably higher roof structure

your red lines show an asymmetrical roof shape and it might be better to only raise a new roof over the rear extension from the original central ridge and keep the front verandah as is but is this the best way to extend your house?? we need to see the original floor plans and a site plan showing dimensions of rooms and the block ...but, adding a 4m extension across the whole back wall may create challenges with windows and plumbing and walls and hallways and an L shape extensions at the back may be better but what is the problem with the front verandah roof...why does it need to be changed

The white roof already at the rear is a skillion roof, we wish to build a deck of the rear the connects to the extension. Wit the front the front awning is rusbbish laser lite sheeting and we wanted to pull the rest of the house forward to align withthe verandah. Then from this wanting to put the whole house including front and rear verandah under the one roof. Im guessing, the more i research, its best to remove the whole roof structure and start again might be cheapest to, guess I was hoping to not have to compromisie the ceiling.

A family member has a house with a gabled end roof similar to your original, it had verandahs running along both long sides. They wanted to extend the front verandah to double it's depth, so the builder cut the old verandah off and then built a new roof that was stepped back over the old front part of the remaining roof so that the pitch of the new roof section would work. So still ending up with a gabled roof

Eseentialy the rooms of the back aren't affected they will just have the veradah built off it, with the front those rooms extend forward so window remain in same position just room become larger. Having a builder or 2 come out next week so i should get some clarity of the options i have and which way to go.

look forward to learning their opinions

following up on my comment above, a rough diagram of what I was trying to explain and probably still need to.

The house I mentioned is a long gable ended rectangle with verandahs running along both long sides. They wanted to double the depth of the Front veranda without changing the whole roof structure but were replacing all the metal roofing. The builder cut off the roof of the front Verandah and then stepped the new roof framing back to acommodate the new width., A new carport was added to the side of the house and was incorporated under this new roof which was extended to the side , which ended up as a skillion . The end of the gables and the new carport had vertical batons attached, a bit post WW2 style. I hope my diagram helps explain

Ah I see, yep that would work for me too,

keeping mind that is just a basic diagram, this style of roof has a name, your builders should know it. I have tried to show this from standing beside the new double carport looking back towards the house, look forward to hearing how you go