ᐅ Renovating an Old Roof or Upgrading Interior Insulation? A Guide for Homes Built Around 1920

Dear Forum,
We are planning to buy a semi-detached house built in 1920 with an extension from the 1960s.
Below are a few drawings / pictures related to the roof:

From an energy standpoint, I want to invest a bit and will have an individual building energy assessment (iSFP) done.
Of course, our budget is limited; otherwise, we would probably do everything.
My thoughts:
- Roof: Either renew the insulation between the rafters plus add internal insulation, or completely renovate the roof after about 4-5 years (see another forum post).

Current rough estimates for a full roof renovation are around 70,000 euros, which is beyond our budget.
I will soon go with a carpenter to assess various aspects.
I fear the assessment might be something like:
"Your planned internal insulation can be done, but due to the complexity it won’t achieve the insulation effect you hope for..."
Others also say that the tiles still look good and the flat interlocking tiles can last a very long time. Unfortunately, I don’t know how old the existing ones are.
A few years ago, the attic floor and probably also the roof slopes in the second floor were insulated with about 15-20cm (6-8 inches) of glass wool (?).

What do you think?
Should we invest the effort to renew the internal insulation for less than 15,000 euros and leave it at that, or would it be better to do the entire roof at some point?
In that context, would you remove the ceiling between the attic and the second floor to make the internal insulation easier and to gain ceiling height (+1.5 meters (5 feet))? Or would the thermal disadvantages of only renewing the internal insulation be higher?

Thanks and best regards,
Sebastian

SebastianHe schrieb:

We are going to buy a semi-detached house built in 1920 with an extension from the 1960s.
Below are a few drawings/images related to the roof:

The details are hard to see (and even harder to recognize) in these thumbnail collages.

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

The entirely new roof with continuous insulation above the rafters is obviously better, as you can build everything according to current standards at once.
I don’t understand why the EL option should cost 15k.
Study the necessary technical knowledge, double up the rafters, add mass to the roof, and go for it.

Yes, that works, thanks. That is the maximum budget I want to allocate for this. I am currently trying to estimate what the different measures might cost me. I think 15,000 (fifteen thousand) will be quite generous with a lot of personal effort, but if most of the work is outsourced, it might not be too much.

Here are a few pictures in better quality:

- The roof has an area of about 120m² (1,290 sq ft).