Do the shell including roof. Partition the living spaces you intend to move into and finish those. Block off other spaces and move into those spaces as you finish them. E. G block off access to unfinished bedroom from the main house. Finish the bedroom from outside and open the access when doing finishing touches.

Another option would be a twin house concept with a connecting corridor. This is to avoid interfering with roof structure of original house when developing second house

That Bungalow rendition looks nice sana.

Stage 1:, get an archetural design and discuss your construction strategy (phased) with the designer. Note: an architectural design of a house, is not a plan but is a conceptualised model of the desired house in details. A plan is an execution strategy where the architectural, structural, mechanical, electrical etc designs, boqs, master /site layout plans are the tools of execution.
Stage 2. Develope an execution plan to cut on wastage and bottlenecks. Eg,it may be difficult to phase out some roof designs, you may not live in a house without basic services like closets, basic sewer infrastructure, electricity connection points etc.
Stage 3. Always have a budget. It's so sad to fail to complete a development for immediate use boy, particularly if credit financed. Renting versus servicing a loan at the same time when u wished to escape from rent can leave u properly quanderied my dear wamugunda.
4. Always delay final touches and top notch painting, expensive fittings, landscaping, expensive floors, ceilings etcs. Construction at times can be messy. Kama utaki kufura kichwa Kama kifaru tafadari.
Stage 5. Set up should be whole. Do not segment set-up. Many errors are recognized at this stage. Early rectification of errors is crucial to a successful project. Designers are aware not every detail can be read from the plan. A non issue element might be so a major issue to a particular site such that other issues becomes mpango wakando when ng'ina wa ciana steps into the scene of. Eg, boundary, soil type, adulation, services, percel shape, design orientation etc
All these will and will oftenly result in change of ur initial instructions. So be ready to engage and cede position on your requirements side and ask for genuine explanations. Designers may take your instructions literally, and leave u up and dry during execution.
Stage 6. Lazima upelekwe chule ya ukaindi,ndio ukuwe mkaindi Kama mkia wangu. Sio ajabu Kila fundi atakuwa na different level of skills and thus breaks to appear in your work messing the whole thing up. Anything that's is not good now will not be good tomorrow, you can not remendy wall construction errors while at texturing stage of construction. Too much effort may go into cleaning, almost making it an activity of it's own. Consistency is key.

Finally, I can give ✋ where need be.

Thanks for this good advice baba. Bulding with loans is suicide jameni. Imagine someone borrowing at 16% and then gets fleeced by the contractor and fundis so the building is not finished (meaning they cannot enter) yet the loan has been used up. And payment demand from the bank ni ile ile plus rent is still eating them alive!#EconomicSuicide Afadhali they build pole pole loan-free hata kama it is two courses of stones per month. Slow but sure. That way they sleep like a baby usiku.

Legally, It's not allowed to live in a building under construction