Flat roof bungalow built in 1970
Built in 1970 means constructed before the first thermal insulation ordinance of November 1977. Most of these buildings were not planned and built according to energetic criteria, but rather according to what the client, architect, and zeitgeist fancied at the time. The energy demand per m² of living space is even higher for bungalows because there is more exterior surface per m³ of air (about 30% more).
Am I wrong or does this initially sound like a lot of work and money that absolutely must be put in?
The "must" has already been relativized by . It makes sense if you want to live in it for another 30-50 years, but in many areas.
Roof: Flat roofs from this era are generally no more critical than today's flat roofs. They are just older and therefore more prone to leaks than brand new ones. There are plenty of examples here in the forum of poorly executed (and poorly designed!) flat roofs. If the roof is watertight now, that is already a sign of reasonable planning and execution. Flat roofs from the 60s/70s were usually cold roofs, i.e., with ventilated insulation. You can imagine it like a ceiling with insulation inside and some air above it beneath the roof covering. The rafters are usually only insulated with 5-6 cm, then there is about 10-20 cm of air, and then the covering with waterproofing. Somewhere inside is the internal drainage. It makes sense here to significantly reinforce the insulation over the full rafter width (20-24 cm) and to renew the waterproofing and roof covering. This is not cheap but costs roughly as much as for a pitched roof: 150-200 €/m², so in the order of 30,000€. You won't recoup that solely from energy cost savings, but it should also be seen as an investment in the structural safety of the house. This intervention, however, produces no dirt or work inside, so it can also be done while the house is inhabited.
Basement: Usually basements are not insulated and sealed with a double bitumen waterproof coating. Mostly, not much of that remains. If the soil is sufficiently permeable (no clay) and there is no accumulating seepage water (e.g., hillside location), there should be no objection to using the basement as storage in the future as well. If large-scale dampness or crystallization deposits are already visible, or even plaster/concrete flaking on exterior walls, the external sealing should be renewed. For a 140 m² bungalow that means a lot of earthworks (a trench around the house 2.5 m deep and over 1 m wide) and a lot of money (about 400€/running meter, though the range is quite large here). If only one wall, e.g. due to hillside location, is affected, the work can be limited accordingly. The work itself is definitely no fun, but those reasonably skilled can do it themselves – however, the earthworks should be done by someone knowledgeable and sloped accordingly. With sufficient ceiling height in the basement (>2 m), insulating the basement ceiling from below makes particular sense for bungalows with full basements. It’s cheap, clean to do, can be done by oneself, and is worthwhile. It also saves an expensive renewal of the screed above!
Water/sewage: Water mostly executed in steel. For safety reasons, I would replace it; it should be easy with a full basement. Sewage must be renewed anyway during bathroom remodeling and is almost free if exposed in the basement.
Electricity: At least renew the fuse box and check the wiring. If there are enough fuse circuits and a clearly sensible distribution, nothing needs to be renewed. Of course, it’s different if you want more sockets/switches.
Windows: From 1970: definitely renew. Triple glazing with warm edge spacer: 300-500€/m² including installation in plastic; fancy stuff like lift-and-slide doors is naturally more expensive. Fixed glazing elements are cheaper. Bungalows from that era usually have large glass fronts; for example, a large double door and the rest fixed glazing is recommended. There are exceptions. Sometimes gigantic fixed glass fronts and a large sliding door made from Meranti or similar wood with an overhang were installed. These still look like new. There, you can just replace the glass for 100€/m² and enjoy it for another 30 years. Glass block walls, if present, should be replaced with normal fixed glazing! They are never load-bearing and can easily be knocked out.
Heating: Depending on age, keep or replace. If gas is available, I would switch to gas in the medium term; otherwise leave as is or install a new condensing boiler. The radiators can remain; newer radiators are more sensible for low heating water temperatures. The pipes are uncritical—they are in the basement and do not rust away or anything. If they do, you will see it directly, and you won’t have large damage or expensive digging work. New heating: 8,000€+, radiators with installation 500€ each.
Walls: All sorts. It rarely pays off to add insulation there additionally. If there is facing brickwork, it is also durable almost forever, so just leave it. Different for timber frame construction, but those were generally not bad at the time. More important is to brick up radiator niches if present and place radiators in front of them. Doesn’t cost much and can be done by oneself using aerated concrete.
Floor: Nice old herringbone or mosaic parquet floor: sand, oil, continue using! Carpet/linoleum/PVC tiles: remove and put in something new (design floor or parquet depending on existing height). Tiles: depending on taste. Screed: leave as is, except in the bathroom—if renewed, do it fully. Cost? 0-100€/m² depending on taste.
A professional must urgently be consulted. I estimate at least 200k€ for the renovation. More likely more.
And you calculated that without pictures, without knowledge of the construction or condition, and without knowledge of the questioner’s needs and requirements? Chapeau! Someone with experience in renovating old buildings who knows your demands should be consulted. Before the purchase, if you have no idea, an expert is recommended (not a KfW salesman, but a neutral expert). You can also consult a KfW salesman afterwards; the important thing first is whether everything is intact and reasonably constructed.