ᐅ Extension of a single-story house in bungalow style with a 30-degree hipped roof

Hello,

We built a single-story L-shaped house in 2016 in a bungalow style, meeting the KFW70 standard. The house has approximately 170 m² (1,830 sq ft) of living space, features a reinforced concrete ceiling, and the roof is a hipped roof construction with an extension made of glued laminated timber at a 30-degree pitch.

Originally, the attic was intended only as a storage area, and its height was sufficient for that purpose. Now, however, we want to convert the attic into a living space, which was not planned for originally, but unfortunately, the height is inadequate.

We have considered two solutions so far: a hydraulic roof lift (which is not an option for us) or increasing the knee wall height. (Currently, there is no knee wall.) At the moment, there is a foldable loft ladder, but we have already planned a larger opening in the reinforced concrete ceiling for a proper staircase (now covered).

Since I am not an expert, I was hoping to find some advice and tips here.

In the first few years, the house developed some settlement cracks in the plaster. Because of this, I would like to have the foundation’s strength checked independently from any structural calculations before we even start planning the extension. How is this possible, and what costs should we expect?

The idea is to "remove" the roof and later reuse the usable parts (wooden beams, tiles, etc.). Then, build up the walls to a height of 1.2–1.4 m (4–4.5 ft) (At what point would this count as a second floor?). After that, the roof would be reinstalled as before. If possible, expansions like large roof windows and a photovoltaic system could be added during this process.

The development plan allows for a second floor, but of course, we would submit a building inquiry/planning permission request beforehand.

If it helps, here are some details about the house:

30 cm (12 inches) gravel filter layer under the base slab, delivered, installed, and compacted.

Base slab made of reinforced concrete C 20/25, 20 cm (8 inches) thick, reinforced with 2 layers Q257 plus necessary additions, on existing gravel sub-base including edge formwork and vapor barrier; delivered and installed.

Ceiling 20 cm (8 inches), complete.

Beams B/H 30/25 cm (12x10 inches) reinforced concrete beams including formwork up to 2.5 m (8 ft) height and reinforcement (150 kg/m³).

Reinforced concrete columns B/H 25/25 cm (10x10 inches), C25/30 concrete including formwork (height below 2.6 m / 8.5 ft) and reinforcement (140 kg/m³).

Aerated concrete masonry 36.5 cm (14 inches) GB 2.

Calcium silicate masonry 17.5 cm (7 inches) thick.

Calcium silicate masonry 11.5 cm (4.5 inches) thick.

Flat lintels made of aerated concrete, 17.5 cm (7 inches) thick.

Precast calcium silicate lintels 17.5/11.5 cm.

Details on the roof construction are as follows:

- Delivery, assembly, and raising of a hipped roof structure with an extension made of glued laminated timber.

- Roof pitch 30 degrees.

- Roof overhang 60 cm (24 inches).

- Supply and installation of galvanized metal fittings.

- Use of a mobile crane during roof assembly.

- Tongue-and-groove boarding 19.5 x 121 mm (¾ x 4¾ inches) installed as a cornice box including substructure, extending to the terrace area.

- Terrace beam made of laminated timber.

We plan to carry out the conversion within 1–3 years with a budget of 120,000 to 150,000 euros. Depending on the possibilities, we would like to add 1–2 large windows with access and possibly a photovoltaic system up to 10 kW peak. (An 18 kW peak system is already installed on a second building.)

We already have a brine geothermal heat pump that provides sufficient performance for the ground floor area. The floor will be equipped with full-surface underfloor heating like the ground floor.

I have attached two pictures. The black-and-white photo shows the terrace and the garage on the left side.

Is our plan feasible? Is the budget roughly sufficient? How can we have the current foundation checked independently of a structural engineer?

Thank you very much for your advice!

Do you want to add a full additional floor above the ground floor? So, in the end, having 340 sqm (3,660 sq ft) of living space? I estimate that the budget might be rather tight. You not only have to pay for building the upper floor but also for dismantling the roof.

Myrna_Loy schrieb:

Are you planning to add an entire floor to the ground level? So that you end up with 340 m² (3,660 sq ft) of living space? I guess the budget might be a bit tight. You’ll not only have to pay for constructing the upper floor but also for dismantling the existing roof.

We would indeed want to add a full additional floor. I also find it hard to imagine that the sections could be separated. The work will probably have to be done as a whole.
Here in the far East, costs will certainly be significantly lower than in metropolitan areas.
Currently, the hourly wage for skilled workers here is still around €31 (about $34) net.

Basically, I would like to have about 100–120 m² (1,075–1,290 sq ft) of living and usable space upstairs.

Is the current heating system designed for the additional area, or do you need a larger one?

hanse987 schrieb:

Is the current heating system designed for the additional area, or do you need a larger one?

Hello,

I mentioned this earlier in the text:

We have a ground-source heat pump that is powerful enough for the floor area. The floor will then be equipped with a full-surface underfloor heating system, just like the ground floor.

We deliberately chose a heat pump larger than necessary during the house construction. After 6 years of heating, cooling, and domestic hot water use, we have a COP of 5. That really was the most sensible decision for the entire house.

That won't work with the budget.
Especially not within 1-3 years.